4x³+16x²+15x.
Volume is l*w*h, so the volume is the product of the 3 lengths here. The "X" side can simply be multiplied by one of the other sides by distributing (multiply each of the two parts by x). That leaves two binomials to multiply, which one can do with FOIL (Mutliply the First of each, the Inner two, the Outer two, and the Last of each, then add them.