Let N,O be functions such that N(x)=2srt(x),O(x)=x^2. What is N(O(N(O(N(O(3))))))? Thank you!

N=2√x, O=x^2

So, just work your way out through the parentheses:
N(O(N(O(N(O(3))))))
N(O(N(O(N(9)))))
N(O(N(O(6))))
...

Or, note that N(O(x)) = 2√(O(x)) = 2√x^2 = 2x

So, you have N(O)^3 = 2^3*3 = 24