Think: To write the equation of a line in slope-intercept form y=mx+b, you must have the slope (m) and the y-intercept (b).
g(0)= -6 tells us that the y-intercept is (0, -6). Thus, b= -6.
(0,-6) and (15,-9) are points on the line. Find the slope of the line. It is:
-9-(-6)
m = ------------- = -3/15 = -1/5
15-0
Thus, the equation of the line in question is y=(-1/5)x - 6.
Important that you understand that g(0)=-6 is also represented by (0,-6), and that -6 is the y-intercept.