Answer
$ \$ 358.84$
Work Step by Step
Recall:
$A = P \left(1+\dfrac{r}{n} \right)^{n t}$
where
$P:$ The principal amount
$r:$ Annual interest rate
$n:$ Number of compoundings per year
$t:$ Number of years
$A:$ Amount after $t$ years
The given problem has:
$P= \$ 300, r=0.12, t = 1.5$
$\text{Compounded monthly} \to n = 12$
Thus, using these values and the formula above gives:
$A = 300 \left(1+\dfrac{0.12}{12} \right)^{12 \times 1.5}$
$A \approx \$ 358.84$