TeX source:

\begin{array}{cl} y=a b^{x} & \text { Write the general form of an exponential equation. } \\ y=3 b^{x} & \text { Substitute the initial value } 3 \text { for } a . \\ 12=3 b^{2} & \text { Substitute in } 12 \text { for } y \text { and } 2 \text { for } x . \\ 4=b^{2} & \text { Divide by } 3 . \\ b=\pm 2 & \text { Take the square root. } \end{array}