Answer
$y=0.5\cdot(0.5)^x$
Work Step by Step
Let $y=ab^x$.
Then our equations are: $2=ab^{-2}$ and $0.25=ab^{1}$.
If we divide the second equation by the first one we get: $0.125=b^3$, thus $b=0.5$. Then $0.25=a(0.5)^1=0.5a\\a=0.5$.
Thus $y=0.5\cdot(0.5)^x$
