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) * 8 + 140 * 2 = 100 * 6 + (100 + 180) * ?.
==> 2840 = 600 + 280 * ?.
==> 2840 - 600 = 280 * ?.
==> ? = 2240 / 280.
==> ? = 8.