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.
(100 + ?) * 2 + ? * 6 = 180 * 4 + (180 + 120) * 2.
==> (100 + ?) * 2 + ? * 6 = 1320.
==> Write without parentheses: 100 * 2 + ? * 2 + ? * 6 = 1320.
==> Simplify: ? * 8 + 200 = 1320.
==> ? * 8 = 1320 - 200.
==> ? = 1120 / 8.
==> ? = 140