Since I'm not sure what equations you are choosing from...
First, find the slope:
m = y2 - y1 = x2 - x1 = -6 - 0 / 0 - (-3) = -6 - 0 / 0 + 3 = -6 / 3 = -2
Now find the slope intercept:
y = mx + b
0 = -2(-3) + b
0 = 6 + b
-6 = b
The final equation should be:
y = -2x - 6
