# complex exponential signal periodicity

I am used to identifying and proving periodic signals in the form: $x[n] = 3cos(\frac{5n}{7} + \frac{ \pi }{4})$ with the 'rational/irrational'
When it comes to this from: $x(t) = e^{(-2+j( \frac{ \pi }{4} ))t}$ I am not sure how to prove it mathematically. I know that it is a signal with
The only step I did for making the signal more clear is: $x(t) = e^{-t}[cos \frac{ \pi t }{4} +jsin \frac{ \pi t}{4}]$