That wasn't bad. It's always irritating to get the hard part and stumble on the simple stuff you know already. Good work, though. It better than the other way around. Reviewing always gets in the way of advancement.
If C1 * V1^3 = C2 * V2^3
And If C2 = 0.72* C1
This gives: C1 * V1^3 = (0.72*C1) * V2^3
Solving for V2 in terms of V1 gives:
Can the question now be answered?