Уравнение: y = kx+b
y1 = kx1+b
y2 = kx2+b  => y1-y2 = k(x1-x2)

y3 = kx3 + b

y1-y3 = k(x1-x3) => k = (y1-y3) / (x1 - x3)
y1-y2 = (y1-y3) / (x1 - x3) * (x1-x2) | *(x1-x3)
(x1-x3)*(y1-y2) = (y1-y3)*(x1-x2)