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.
(180 + 120) * 6 + 120 * 2 = 120 * ? + (120 + 140) * 6.
==> 2040 = 120 * ? + 1560.
==> 2040 - 1560 = 120 * ?.
==> ? = 480 / 120.
==> ? = 4.