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.
(140 + ?) * 4 + ? * 2 = 140 * 8 + (140 + 120) * 2.
==> (140 + ?) * 4 + ? * 2 = 1640.
==> Write without parentheses: 140 * 4 + ? * 4 + ? * 2 = 1640.
==> Simplify: ? * 6 + 560 = 1640.
==> ? * 6 = 1640 - 560.
==> ? = 1080 / 6.
==> ? = 180