15: The numerator is clear, the leading term is n.

Concerning the denominator, you should have started with individual cubic roots. Inside there is n² prevailing over "+n" and "-n", so both cubic roots involved in the denominator are of the type (n²)^1/3 = n^2/3.

Using this one observe that all three terms in the denominator are of the type (n^2/3)² = n^4/3. This is then the leading term for the denominator, fortunately the terms are added so there will be no trouble. Factor the dominant terms out and see what happens.

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