The scale is balanced. This is the case if the distances * weights are the same on both sides of the scale: (A + B) * weigth 1 + B * weight 2 = C * weigth 3 + (C + D) * weight 4.
(120 + 140) * 4 + 140 * 8 = 120 * ? + (120 + 120) * 4.
==> 2160 = 120 * ? + 960.
==> 2160 - 960 = 120 * ?.
==> ? = 1200 / 120.
==> ? = 10.