the perimeter p of the rectangle with length l and width w is:
p = 2l + 2w
that is, two times its length and two times its width, lets solve for the width w and substitute known data:
p = 2l + 2w
2w = p - 2l
w = (p - 2l)/2
w = (56 - (2*12))/2
w = (56 - 24)/2
w = 32/2
w = 16
therefore the width of the rectangle is 16 in