The volume of the cube would be S^3, where S is the length of the side.
Volume = (5a + 4b)^3
Which becomes:
(5a+4b)(5a+4b)(5a+4b)
Multiply the first two sets of parenthesis by each other:
(25a^2+40ab+16b^2)(5a+4b)
Now multiply that by the 3rd set of parenthesis to get:
125a^3+300a^2b + 240ab^2 + b^3