This is pretty obviously
[tex]f(x) = 5^x[/tex]
If it wasn't obvious we can find the common ratio r by dividing successive values:
5/1=5
25/5=5
125/25=5
625/125=5
That's the common ratio. We multiply by the first term, but the first term is one, so that has no effect.
[tex]f(x)=5^x[/tex]