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 + 200) * 2 + 200 * 10 = 180 * 6 + (180 + 100) * ?.
==> 2760 = 1080 + 280 * ?.
==> 2760 - 1080 = 280 * ?.
==> ? = 1680 / 280.
==> ? = 6.