Answer
y = 3x + 17
Step-by-step explanation
If you you want to find the equation of a line you require a point on that line and is gradient.
gradient = 3
a point on the line is (-8, -7)
Chose another general point (x, y)
gradient = (change in y)/(change in x)
3 = (y- -7)/(x- -8)
3 = (y+7)/(x+8)
3(x+8) = y+7
3x+24 = y+7
y = 3x + 24 - 7
y = 3x + 17