Question

How could the graph of an exponential function be used to determine an `a` value?

Answer 1

I assume you mean the "A" as in the initial value of `y=Ae^(rt)`? `y`-intercept

The "a" value occurs at `t=0`, so `e^0=1`, therefore `y=A` is `(0,A)`, the `y`-intercept.