Let the coil inductance be L and the capacitance of the capacitor be C.
When the ideal coil and an ideal capacitor are connected in series,
Xs = L + C
When the ideal coil and an ideal capacitor are connected in parallel,
Xp = (L/C) / (L + C)
Substituting Xs into the equation for Xp,
Xp = (L/C) / Xs
C = L / Xs • Xp