Given,
Height of the rectangular prism (h) = 4 inches
Width of the rectangular prism (w) = 12 inches
Since the prism can hold 432 cubic inches of water,
Volume of the rectangular prism (V) = 432 inches³
Formula for Volume of a rectangular prism (V) is,
V = whl
I am assuming you are looking for the third side, the length (l) of the prism.
From the above formula for volume,
Length of the prism (l) = V ÷ wh = 432 ÷ (4x12) = 432 ÷ 48 = 9 inches