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) * ? + 140 * 2 = 140 * 2 + (140 + 100) * 8.
==> 320 * ? + 280 = 2200.
==> 320 * ? = 2200 - 280.
==> ? = 1920 / 320.
==> ? = 6.