exam Maths Puzzle Numerical Ability

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i cant figure out thi problem and as wondering if you could help me out on it.
f[x]=-xSq.+9x-5 there is four answers a[-9over2,-263over4] b[9,5 ] c[9over2,61over4] d[9over2,101over4] i think it might be c but iam stuck RATIONALIZE THE DENOMINATOR
1 DIVIDED BY THE CUBE ROOT OF a PLUS THE CUBE ROOT OF b.