Precalculus, Chapter 10, 10.1, Section 10.1, Problem 17

The given slope is m = -1 .
Take note that if the slope of the line is given, to determine the angle of inclination, apply the formula:
tan theta = m
where theta is the angle measured counterclockwise from the positive x-axis going to the right of the line. And its range of values is from 0 to 180 degree only (0^o lt= theta lt=180^o) .
Plugging in the value of m, the formula becomes:
tan theta = -1
Then, take the inverse of tangent to isolate theta.
theta =tan ^(-1)
theta = -pi/4 rad =-45^o
Take note that when the computed value is negative, to get value of angle of inclination in the interval 0^o lt= thetalt=180^o , add 180 degree (pi rad).
theta = -pi/4 + pi = (3pi)/4 rad
theta = -45^o + 180^o = 135^o
Therefore, in radians, the angle of inclination is (3pi)/4 rad. And in degree, the angle of inclination is 135^o .