Help with transformation of formula

Hey guys, I'm struggling to answer this problem:

http://imgur.com/a/WOCra

So this is how I'm looking at it

CR = V1+V2/V2 CR = 400 + X / X CR * X = 400 + X CR* X -400 = X

And then I get a bit lost...What am I meant to do with the 'desired' compression ratio?

Any help would be greatly appreciated