In a series circuit R = $300\Omega$, L = 0.9H, C $=20\mu F$ and $\omega$ = 1000 rad / sec. The impedance of the circuit is 

$R=300\Omega L=0.9HC=20\mu F$

$\omega =1000rad/\sec$

$z=\sqrt{R^2+{\left(X_L-X_C\right)}^2}$

$X_L=\omega L=1000\times 0.9=900$

$X_C=\frac{1}{\omega C}=\frac{1}{1000\times 20\times {10}^{-6}}=500\Omega$

$Z=\sqrt{{\left(300\right)}^2+{\left(900-500\right)}^2}$

$Z=\sqrt{{\left(300\right)}^2+{\left(400\right)}^2}=500\Omega$