Answer
$f(t) = a(1 + r)^t = a \cdot b^t$ for growth ($b > 1$, where $r$ is the percent growth rate as a decimal), and $f(t) = a(1 - r)^t = a \cdot b^t$ for decay ($0 < b < 1$).
Explanation
The parameter $a$ is the initial value at $t = 0$ (the y-intercept), and $b$ is the growth or decay factor per unit time.