I really don't see a way to correctly simplify this fraction.
3n^2 + n - n^2
I got 3n
2n^2 + n
This is the final answer because the 3n^2 and the -n^2 are like terms that come together to make 2n^2 the + n is still leftover and therefore added to the end of the formula. You the n and n^2 are not like terms meaning that they can't be added, i think that was your mistake