Total volume is given. You need to find the volume of the left box in order to solve for its height. In order to do that just find the volume of the other two boxes and subtract from the total volume. The middle box has volume of (10)(10)(16)= 1600. The right box has volume (4)(10)(8)= 320. Now subtract. 2400-1600-320= 480. This is the volume of the left box. Now set it equal to the length width and height of the that box. 480=6hw. We know w=10 from given. So h = 480/(6)(10). h=8