side length of a smaller cube = x
volume of a smaller cube = x³
side length of a larger cube = 2x
volume of a larger cube = (2x)³ = 8x³
x³ + 8x³ = 72
9x³ = 72
x³ = 72/9
x³ = 8
x = ∛8
x = 2 cm ← side length of a smaller cube
2x = 2*2 = 4 cm ← side length of a larger cube
