African-American Lit III: The Harlem Renaissance Jean Toomer, Cane Hughes, “The Negro Speaks” (D2027); “The Weary Blues” Hurston, “Gilded Six Bits” McKay, “The Harlem Dancer” The Political 1930s Dos Passos, USA Steinbeck, “The Leader of the People” (D2049-61) Contemporary Poetry Ginsberg, Howl (E2574-84), “A Supermarket in California” Plath, “Daddy” (E2704-6); “Lady Lazarus” Bishop, “The Fish” (E2167), “The Moose” Lowell, “Skunk Hour” (E2405-7), “For the Union Dead” Fiction in the Fifties Bellow, “Augie March” O’Connor “Good Country People” Cheever, “The Swimmer” The 60s Pynchon, “Entropy” LeGuinn, “Schrodinger’s Cat” Vonnegut, Slaughterhouse Five Minimalism & Postcolonialism Carver, “Cathedral” Reed, “Neo-HooDoo Manifesto” Lahiri, “Sexy” Using the readings above, explain whether you think American literature grew more positive or more negative. Is your answer based on whether you think it is more optimistic or pessimistic or neither? Develop your answer using examples from at least three of the writers or poets from the last half of the course.

This is a subjective question—that is, it is based on your opinion. I will only give you my opinion with the expectation that you will use it as a guide in developing your own.
The literature that we best remember is neither strictly optimistic nor strictly pessimistic. Instead, it explores the complexity of the human condition, its triumphs and its disappointments.
The best way to consider the works that you have mentioned is to regard their purposes in their respective decades. Langston Hughes's poem "The Negro Speaks of Rivers" is an ode to African American history and a reminder that it stretches back further than slavery and the advent of the United States. Jean Toomer's Cane is a motley work that explores storytelling in various forms, including vignettes, and it was unique at the time for the central place that it provided to black women. Contrary to popular Harlem Renaissance literature and works which preceded it (I am thinking particularly of Charles Chesnutt's novels), it did not cast "the tragic mulatto woman" as a black female protagonist; it cast many types of women. It was, thus, more modern in its outlook, given its emphasis on multiplicity and its vision of an Old South in collapse due to the invasion of the boll weevil.
Works from the 1930s were more socially oriented and more directly concerned with events that impacted the masses, such as the Dust Bowl and the Great Depression. Some of this was due to the influence of the Freedom Writers' Project, a subdivision of the Works Progress Administration, which employed writers to work on various projects. There was a greater sense of being a part of a common collective in the 1930s, and the literature reflected that. Thus, though it was a very pessimistic time, it was positive in terms of its collective political efforts.
The period after the Second World War saw the popularity of psychoanalysis and Freudian theory, as well as an alienation with postwar expectations of conformity. Outwardly, the 1950s were very sunny and optimistic, but literature from the Eisenhower era painted another picture, one of alienation and dismay with the nuclear age. Flannery O'Connor's short story "Good Country People" distorts the idealized vision of rural America and the presumed wholesomeness of its people. Allen Ginsberg's poem "Howl" illustrated the frustrations of youth and the seedier side of the halcyon fifties:

I saw the best minds of my generation destroyed by madness,
Starving, hysterical, naked, dragging themselves through the Negro streets at dawn looking for an angry fix.

Certainly, this is a pessimistic picture of madness and drug addiction. However, it is also a rallying cry and an acknowledgment of those who did not fit into mainstream society.
Because you are dealing strictly with American literature, in your conclusion, it might be helpful to talk about how these influences have impacted literature and cultural understanding. You might want to discuss how Toomer and Hughes have expanded ideas of race or how Southern Gothic literature (O'Connor) transformed our vision of the South—for better or worse. Alternatively, you could consider how Beat literature (Ginsberg) exposed the hypocrisies of 1950s America. There are many directions in which you can go with this.