5n^2
You can divide both 50 and 45 by 5, and there are no numbers with a greater value that will be a factor of both numbers.
As for the variables, both n^2 and n^3 are divisible by n^2. Some people would say that the gcf is just n, but that's wrong because 1 is a factor of n^2. Obviously, that means that you could multiply (n^2)x(1) to get n^2. You could also multiply (n^2)x(n) to get n^3.
Please don 't hesitate to ask for further explanation.