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