Calculate ϴ dependent on n for an algorithm with constant c
I calculated the amount of operations to (c-n)*(n-1)


Not quite. If $n &gt; c$ then the cost is not negative: it should be $(n-1) \max(c-n, 0) = \Theta(n) \max(c-n, 0)$. When $n$ is large (and in particular when $n &gt; c$) what does this reduce to?