# Proving that a problem is in NP

I have an assignment in which the problem, $D$, is simple but, once found, easy to check. Is it enough to prove that a solution $x$ can be checked in polynomial time to prove that $D \in NP$? (The problem $D$ is a decision problem)

Edit: Changed $P$ to $D$

2. there must be some polynomial $f$ such that solutions to instances of length $n$ have size at most $f(n)$.
• @NyfikenGul Yes, it will. But there's no systematic way of finding what $f$ is. Mathematical proof is a creative act, not a mechanical process. – David Richerby Feb 23 '17 at 19:50