Hello,

You get n as the derivative.

Which is always positive.

Thus the likelihood function is always increasing.

In order to maximise the likelihood function, make a the greatest possible.

But you know that

Since the xi's are observations, they're already realised. So find a with respect to xi. Such that it's the greatest possible.

Would you agree that ?

A similar reasoning is made for the MLE of a uniform distribution

I've seen that on a wikipedia (can't remember in which language), so you should find an example over the internet !