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) * 6 + 100 * 10 = 100 * 6 + (100 + 100) * 8.
==> (? + 100) * 6 + 1000 = 2200.
==> (? + 100) * 6 = 2200 - 1000.
==> (? + 100) = 1200 / 8.
==> ? + 100 = 200.
==> ? = 200 - 100.
==> ? = 100.