Question

How do i graph y=-x-21

Answer 1

Take 2 points  :- 
if x = 0 y = -0-21 = -21 ,   point A = (0,-21)

if y = 0  0 = -x - 21 so x = -21,   point B =  (-21,0)

Plot points A and B and draw a line through these 2 points.

Answer 2

You would put -21 on bottom right on the x axis and -x on the top left on the y axis