The formula for the volume of a box (assuming that it is in the shape of a cube) is V=B^3.
12=B^3
Now, do the cube root of 12 to find each dimension, all dimensions will be equal as it is a cube.
B= about 2.29 (when doing on a calculator keep full answer and do not round)
Now to find the volume of box B, multiply each dimension by 5
5(2.29...) times 5(2.29...) times 5(2.29...)
=1,500 m^3
Hope this helps!