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.
(100 + 140) * 4 + 140 * ? = 200 * 6 + (200 + 100) * 2.
==> 960 + 140 * ? = 1800.
==> 140 * ? = 1800 - 960.
==> ? = (840 / 140.
==> ? = 6.