k1 = (y2 - y1) / (x2 - x1);
c1 = y1 - (k1 * x1);

k2 = (y20 - y10) / (x20 - x10);
c2 = y10 - (k2 * x10);


x = (y1 + c1) / k1;
y = k2 * x + c2;