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.
(? + 200) * 4 + 200 * 6 = 160 * 6 + (160 + 120) * 6.
==> (? + 200) * 4 + 1200 = 2640.
==> (? + 200) * 4 = 2640 - 1200.
==> (? + 200) = 1440 / 6.
==> ? + 200 = 360.
==> ? = 360 - 200.
==> ? = 160.