First we need the slope, which we can get using (y2 - y1) / (x2 - x1)
Plug in our coordinates and we get
(15 - 12) / (2 - -4)
3 / 6
1 / 2
So, our slope is 1/2.
Then, we take 1/2 of one of our x values, say, 2, and we get 1. How do we go from 1 to 15? We add 14. So our y - intercept is 14.
All this plugged into y = mx + b gives us the equation y = 1/2 x + 14