Why not try to set instead of in series expansion of and then multiply each term of the series by ?...
Knowing the sum of an infinite convergent geometric series to be where a is the first term and r is the common ratio, compare this to what they have given you.
For a is equal to x^3 and r is equal to -x^2. The first term will be x^3 and then multiply by -x^2 to generate the rest of the terms.
The power series representation of will be provided that for the first term n=0
Nevermind, looks like chisigma beat me to it.