Answer
14 watts.
Work Step by Step
Use the definition of sound level. The change in sound level depends logarithmically on the increase in intensity, and thus, on the emitted power.
$$\beta = 10log \frac{I_f}{I_o} $$
$$\Delta \beta = 10log \frac{I_f}{I_o}-10log \frac{I_i}{I_o}=10log \frac{I_f}{I_i} $$
Use the given information.
$$ -12dB=10log\frac{P_{15\;kHz}}{225\;W} $$
$$ -1.2=log\frac{P_{15\;kHz}}{225\;W} $$
$$ 10^{-1.2}= \frac{P_{15\;kHz}}{225\;W} $$
$$ P_{15\;kHz}=(10^{-1.2})225\;W=14\;W $$
