Persamaan garis yang melalui titik (3, 1) dan (4, 4)

(y - y1)/(y2-y1) = (x-x1)/(x2-x1)
(y-1)/(4-1) = (x-3)/(4-3)
(y-1)/3 = (x-3)/1
y-1 = 3x - 9
y = 3x - 8