Just to expand on the original question. I have to determine positive contants c1, c2, n0 such that:

c1n^2 <= 1/2n^2 - 3n <= c2n^2

After dividing by n^2 I get:

c1 <= 1/2 - 3/n <= c2

Now how can i get those constants? Is it just simply guess work/trial and error?