Given that, y = mx + b ...... (1) then the point-slope form make sense as a name for, y - y1 = m(x-x1) .......(2) Equation (1) we get, y = mx + b b = y - mx ....... (3) m is a slope b is the y-intersect (x1,y1) is the points Solve the equation (2), y - y1 = m(x - x1) y - y1 = mx - mx1 y = mx - mx1 + y1 y = mx + (y1 - mx1) y = mx + b ......... (4) (use equation (3)) Therefore we prove the y = mx - b