McDougal Littell Algebra 2, Chapter 3, 3.1, Section 3.1, Problem 44

Given the system of equations y = -5 - x and x + 3y = -15. Each of the equations represents a straight line.
The graph of the two is:

From the graph, the point of intersection of the two lines is (0, -5)
The equations can be solved by substituting y = -5 - x in x + 3y = -15
x + 3*(-5-x) = -15
x - 15 - 3x = -15
-2x = 0
x = 0
As y = -5 - x. it is equal to y = -5.
This confirms the result obtained from the graph of the two equations.