(a) Here, $L = 50\ \mu H = 50 \times 10^{- 6}H$
$C = 1\ nF = 10^{- 9}F$
$f_{c} = \frac{1}{2\pi\sqrt{LC}} = \frac{1}{2 \times \frac{22}{7}\sqrt{50 \times 10^{- 6} \times 10^{- 9}}}$
$= \frac{7 \times 10^{7}}{44\sqrt{5}}\ Hz$
$= 711.5\ Hz \approx 712\ kHz$