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 + 100) * 2 + 100 * ? = 120 * 6 + (120 + 100) * 2.
==> 560 + 100 * ? = 1160.
==> 100 * ? = 1160 - 560.
==> ? = (600 / 100.
==> ? = 6.