The general equation is y=mx+c
Where m is the gradient and c is a constant (the Y intercept)

You can work out m by constructing a triangle between the two points, (see pic). The height/width gives you the gradient, so in this case it would be 285/140 (just over 2).
You are then left with y=2m+c and you can work out C by putting in some values for x and y (from your points). I'll use 10 and 15 since they're smaller.
15=10*2+c
15=20+c
c=-5 (roughly since m isn't exactly 2)
so the equation is y=2m-5