Chapter 9 - Section 9.2 - Exponential Functions and Models - Exercises - Page 643: 50

$f(x)=90(\displaystyle \frac{1}{3})^{x}$

When $x$ increases by $1,\quad f(x)$ decreases by factor 3 (multiplied by $\displaystyle \frac{1}{3}.$) $\Rightarrow b=\displaystyle \frac{1}{3}$ $f(0)=Ab^{0}=90\Rightarrow \quad A=90$ Thus, $f(x)=90(\displaystyle \frac{1}{3})^{x}$