Since this is the calculus forum, differentiate and set the differentiated function to zero and solve for x. Justify the solution is a maxima.
He said solve for x! You don't "put in" any value of x.
(Although, here, since you are given a value of x, x= e, you could put that in and see if the derivative is 0 there. You might also want to check that the second derivative is negative there (second derivative test) or that the first derivative is positive for x< e and negative for x> e (first derivative test).