Hi, I was given the following question, and I am not entirely sure how to approach it.

Find the constant a such that

lim x->infinity ((x+a)/(x-a))^x=e

I have tried applying l'hopitals rule, and substituting x=1/y so that the limit becomes

lim y->0 ((1+ay)/1-ay))

However, I keep getting stuck. Guidance would be much appreciated.

Thank you very much!