First rectangle:
Has 3 cubes in each 5 layers, and imagine it, so we have all dimension needed which is:
l=3 unit
w=1 unit
h=5 unit.
V=l×w×h
V=3×1×5
V=15 cubic inches.
Second rectangular:
Doing the same like that to get:
l=5 unit
w=1 unit
h=3 unit
V=15 cubic inches. As a result, there are no prism that are greater to each other, which mean that they have the same volume of 15 cubic inches. Hope it help!