slope = (4-20)/(8-1) = -16/7

Question

I need to plug the slope into y=mx+b with one of the points for x and y and then solve for b.

Answer 1

if the point is (h,k) then you have
k = -16/7 h + b
b = k + 16/7 h
So, if you have the point, say, (7,4) then
b = 4 + 16/7 * 7 = 20