the given points are
(0,-6) and (15,-9)
first find the slope:
(y2-y1)/(x2-x1) = (-9- -6)/(15-0) = -3/15 = -1/5
plug in one of the points and the slope into point slope form:
y-y1 = m (x-x1)
y+6 = -1/5 (x-0)
solve for slope intercept form
y+6 = -1/5x
y = -1/5x - 6
