Answer
$y=64\cdot(0.5)^x$
Work Step by Step
Let $y=ab^x$.
Then our equations are: $8=ab^3$ and $2=ab^5$.
If we divide the second equation by the first one we get: $0.25=b^2$. Thus because $b$ is positive: $b=0.5$. Then $8=a(0.5)^3=0.125a\\a=64$.
Thus $y=64\cdot(0.5)^x$
