Assuming that you meant x++ rather than x--, the complexity is $\Theta(nk)$. Since $k = 300$, you can simplify that to $\Theta(n)$. As Ran G. mentions, since $n = O(n^2)$, then your loop is also $O(n^2)$. It's also $O(2^n)$. But the tight analysis is $\Theta(n)$.