Answer
$12 e^{-0.1054\ t}$
Work Step by Step
Recall the property $(a^m)^n=a^{mn}$, so the equation can be rewritten as $$Q=12(e^{\ln (0.9)})^t ~~~~(1)$$
Evaluating $\ln 0.9$ yields $-0.1054$.
Therefore, the equation (1) can be written as $Q=12 (0.9)^t=12 e^{-0.1054\ t}$