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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