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 + ?) * 6 + ? * 4 = 180 * 6 + (180 + 180) * 4.
==> (120 + ?) * 6 + ? * 4 = 2520.
==> Write without parentheses: 120 * 6 + ? * 6 + ? * 4 = 2520.
==> Simplify: ? * 10 + 720 = 2520.
==> ? * 10 = 2520 - 720.
==> ? = 1800 / 10.
==> ? = 180