The phase between the voltage and the current is given as
$\begin{array}{ll}
& \tan \phi=\frac{X_L-X_C}{R} \\
\therefore \quad & \tan 45^{\circ}=\frac{X_L-X_C}{R} \\
\therefore \quad & 1=\frac{X_L-X_C}{R} \\
\therefore \quad & R=X_L-X_C \\
\therefore \quad & X_L=X_C+R
\end{array}$