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.
(120 + 120) * ? + 120 * 2 = 120 * 4 + (120 + 160) * 6.
==> 240 * ? + 240 = 2160.
==> 240 * ? = 2160 - 240.
==> ? = 1920 / 240.
==> ? = 8.