Chapter 31 - Electromagnetic Oscillations and Alternating Current - Problems - Page 937: 36a

$R = 490~\Omega$

On the graph, we can see that $X_C = 100~\Omega$ when $\omega_d = 250~rad/s$ Also, we can see that $Z = 500~\Omega$ when $\omega_d = 250~rad/s$ We can find $R$: $Z = \sqrt{R^2+X_C^2} = 500~\Omega$ $R^2+X_C^2 = (500~\Omega)^2$ $R^2 = (500~\Omega)^2-X_C^2$ $R = \sqrt{(500~\Omega)^2-X_C^2}$ $R = \sqrt{(500~\Omega)^2-(100~\Omega)^2}$ $R = 490~\Omega$