21: You should get
This new limit - a ratio with powers and roots - is a typical example from the box "polynomials and ratios with powers". We already saw that the cubic root behaves like n when n grows large, so both the numerator and the denominator have the leading power n2. You can factor it out and work out what is left, in the denominator it is actually easier to first factor leading powers from the roots and only then pull them out.
Another approach is to cancel n2 in the fraction, which here might be a bit more work due to the roots.