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.
(200 + ?) * 8 + ? * 2 = 180 * 6 + (180 + 140) * 6.
==> (200 + ?) * 8 + ? * 2 = 3000.
==> Write without parentheses: 200 * 8 + ? * 8 + ? * 2 = 3000.
==> Simplify: ? * 10 + 1600 = 3000.
==> ? * 10 = 3000 - 1600.
==> ? = 1400 / 10.
==> ? = 140