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.
(140 + 100) * 10 + 100 * ? = 100 * 2 + (100 + 140) * 10.
==> 2400 + 100 * ? = 2600.
==> 100 * ? = 2600 - 2400.
==> ? = (200 / 100.
==> ? = 2.