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 + 140) * 6 + 140 * ? = 140 * 2 + (140 + 100) * 8.
==> 1920 + 140 * ? = 2200.
==> 140 * ? = 2200 - 1920.
==> ? = (280 / 140.
==> ? = 2.