If $y = e^{ax}$ and $\frac{d^2y}{dx^2} = 16y$, what are the possible values of $a$?

Explanation

1. 01

   $\frac{d^2y}{dx^2} = a^2e^{ax} = a^2y$.

2. 02

   So $a^2 = 16$, and $a = 4$ or $a = -4$.