Answer
$y= e^{\frac{x}{3}\ln \frac{1}{4}}$
Work Step by Step
We have $y=ae^{bx}$
Set $(x,y)= (0,1)$ to compute $a$
So, $1=ae^{b(0)} \implies a=1$
Further, we have $y=ae^{bx}$
Set $(x,y)= (3,\dfrac{1}{4})$ to compute $b$
So, $\dfrac{1}{4}=ae^{3b} \implies \dfrac{1}{4}=e^{3b}$
and $3b =\ln \dfrac{1}{4} \implies b =\dfrac{1}{3} \ln \dfrac{1}{4}$
Now, the exponential decay model is: $y= e^{\frac{x}{3}\ln \frac{1}{4}}$
