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 + 140) * ? + 140 * 4 = 180 * 6 + (180 + 200) * 2.
==> 320 * ? + 560 = 1840.
==> 320 * ? = 1840 - 560.
==> ? = 1280 / 320.
==> ? = 4.