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.
(? + 160) * 4 + 160 * 6 = 140 * 6 + (140 + 120) * 6.
==> (? + 160) * 4 + 960 = 2400.
==> (? + 160) * 4 = 2400 - 960.
==> (? + 160) = 1440 / 6.
==> ? + 160 = 360.
==> ? = 360 - 160.
==> ? = 200.