Answer
2.36 MHz.
Work Step by Step
$X_L=\frac{V}{I}=\frac{12.0V}{1.80\times10^{-3}A}=6667\Omega$
Solve $X_L=2 \pi fL$ for the frequency.
$$f=\frac{X_L}{2 \pi L}=\frac{6667\Omega }{2 \pi (0.450\times10^{-3}H)}=2.36\times10^6 Hz$$
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