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) * 10 + 140 * 6 = 140 * 6 + (140 + 180) * ?.
==> 4040 = 840 + 320 * ?.
==> 4040 - 840 = 320 * ?.
==> ? = 3200 / 320.
==> ? = 10.