Drag coefficeant rearranging equations problem

Hi all, thanks for you time...

I'm trying to work out how the answer for this problem is obtained:

Q. The drag coefficient for an aircraft is reduced by streamlining.

For the same power output and frontal area by what percentage is the flight speed increased if the drag coefficient is reduced by 18%.

A. The expression of power through drag P=DxV and the power remains the same so that P1 = P2 (i.e D1 x V1 = D2 x V2)

Now replacing D1 and D2 with the formula for drag and cancelling like terms I am left with (coefficient of drag) C1 x V1^3 = C2 x V2^3

This is where I get stuck, C2 is equal to 0.72 x C1 (-18%) which obviously is input into the above equation but I cannot obtain the answer, which I believe to around a 7% speed increase.

Thankyou

Tom