The Hired Muscle quest is obtained from Farkas in Whiterun.
After the preliminary induction into the Companions, Farkas foists this quest upon you. You simply need to intimidate a randomly-generated NPC; no killing.
Find your target, engage them in a fight and, after defeating them, return to Farkas for your reward.
In some cases, the NPC you are assigned to intimidate will be inaccessible, as they are either dead or in jail or surrounded by scores of other NPCs who will jump to their defense.