Answer :

Formula for pyramid's volume: V = 1/3 * l² * h

Knowing that V = 560, l = 10 and h = 14, we can write it down as:

560 = 1/3 * 10 * w * 14
560 = 3 1/3w * 14
560 = 10/3w * 14
560 = 140/3w                      / * 3 (both sides)
1680 = 140w                       / ÷ 140 (both sides)
w = 12

Doublecheck:

560 = 1/3 * 10 * 12 * 14
560 = 1/3 * 120 * 14
560 = 1/3 * 1680
560 = 560

So it's correct :)