Let f(x)=3^x, what is the value of f(x+2)?

$f \left(x + 2\right) = {3}^{x + 2}$

Explanation:

In these kind of questions, we replace the "x" term with what is within the brackets. So in this question, we have:

$f \left(x\right) = {3}^{x}$

and we're looking for $f \left(x + 2\right)$, so we replace the $x$ with $x + 2$, so we have:

$f \left(x + 2\right) = {3}^{x + 2}$