Answer
$y=0.25\cdot(6)^x$
Work Step by Step
Let $y=ab^x$.
Then our equations are: $9=ab^{2}$ and $324=ab^{4}$.
If we divide the second equation by the first one we get: $36=b^2$. Thus because $b$ is positive: $b=6$. Then $9=a(6)^2=36a\\a=0.25$.
Thus $y=0.25\cdot(6)^x$
