Volume of a square = (s)(s)(s) and s=5a+4b
Therefore: (5a+4b)(5a+4b)(5a+4b)=
(25a^2+40ab+16b^2)(5a+4b)=
(125a^3+200a^2b+80ab^2+100a^2b+160ab^2+64b^3)= (125a^3+300a^2b+240ab^2+64b^3)
You just multiple the first time the second and then do so again with the combination of the first two times the second. A little cleaning up and you are left with an equation in terms of a and b.
