#### Answer

$f(x)=5^x$

#### Work Step by Step

First, we have to find the function, by calculating the base, $a$.
If the function contains this point (3,125), then $f(3)=125$, this means:
$a^3=125$
We take the cube root of both sides:
$a=5$
Therefore, with $5$, the function is $f(x)=5^x$.