a)Looking at the power dissipated by the resistor,
we want Pmax/Pmin = (Imax/Imin)2 = (Zmax/Zmin)2 = 5.
Solve for L in
5 = [R2+(omega L)2] / R2.
b) Set up a resistive voltage divider and repeat the calculation above.
Thanks go to Reggie Kee for pointing out errors in original hint.