You already know the slope is 3 and you have the point, so you would plug it into y = mx + b
-2 = y
3 = m
-5 = x
Plug in:
-2 = 3(-5) + b
Solve:
-2 = -15 + b
-2 (+15) = -15 + b (+15)
13 = b
Now you have b, so you can form the equation where the slope is 3x and the y-intercept is 13.
Final answer: y = 3x + 13