Critical points are found by makin' do it.
First you find the derivative of it, by using the product rule. If you don't know how to do that then you should start paying attention in class.
I'm almost positive this is right, the derivative of the exponent of e may be wrong. So if it is, can someone kindly point that out.
You can carry on from here. Clean it up a bit to make life easier, then set to equal zero and substitute x then solve for a.