In the given circuit the resistance R can be varied. Battery having emf 18 volt is having internal resistance $3\text{Ω}$. The value of  R is selected such that power dissipated in it is maximum. Now  $6\text{Ω}$ resistance is replaced by an inductor having inductance  $L=6\text{ mH}$ so circuit will appear like the circuit shown in diagram given below

Find time constant for above circuit in milliseconds.