Question

5^(-x+3) = 64

I have only had exponents to the x, not with +3 too.

Answer 1

Don't know if there is a typo.
Solution is in integers if it were:
2^(-x+3) = 64, or
4^(-x+3) = 64

For the given problem, use the rule of exponents:
a^x * a^y = a^(x+y)

So
5^(-x+3) = 64
5^(-x) * 5^3 = 64
5^(-x) = 64/125
and use a^(-x) = 1/a^x
5^x = 125/64

Can you take it from here?