y=mx+b slope = rise/run = (0-a)/(b-0) = -a/b y=(-a/b)x + B (where B is the y-intercept, not b from the point (b,0) Substitute one of the point into the equation to solve for B. (0,a)→ a = (-a/b)0 + B a = B So rewrite the equation to be: y=(-a/b)x + a