Yes, you can take C = 61. However, k = n log(n) is incorrect because k has to be a concrete number that does not depend on n, just like C. You can pick k as the smallest number such that for n ≥ k all of the following inequalities, which you used, hold:

n ≤ n^3

n log(n) ≤ n^3

n^2 ≤ n^3

Can you find a k like this?