What is a proof in calculus?

A proof in calculus is when it will make a statement, such as: If y=cos3x, then y'''=18sin3x. Then it will tell you to do a proof. This means you have to solve the equation step by step, coming to the solution, which should be the same as in the statement. If you do come to the same answer as in the statement, then you just correctly did a calculus proof.