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) * 2 + 180 * 4 = 120 * 6 + (120 + 180) * 2.
==> (? + 180) * 2 + 720 = 1320.
==> (? + 180) * 2 = 1320 - 720.
==> (? + 180) = 600 / 2.
==> ? + 180 = 300.
==> ? = 300 - 180.
==> ? = 120.