first we find the slope
(0,0)(4,-4)
slope = (y2 - y1) / (x2 - x1)
slope = (-4 - 0) / (4 - 0)
slope = -4/4 = -1
y = mx + b
slope(m) = -1
(4,-4)...x = 4 and y = -4
now we sub...we r looking for b, the y intercept
-4 = -1(4) + b
-4 = -4 + b
-4 + 4 = b
0 = b
so your equation is : y = -1x + 0....or y = -x