Answer
$f(t)=\frac13\left(9\right)^t$
Work Step by Step
$$f(t)=\left(\frac13\right)^{1-2t}=\left(\frac13\right)^{1}\cdot \left(\frac13\right)^{-2t}=\dfrac13\left(\left(\frac13\right)^{-2}\right)^t=$$
$$=\dfrac13\left(\left(3\right)^{2}\right)^t=\dfrac13\left(9\right)^t$$
