Carmen, who runs the Eager Thighs Brothel (#1) will give you this quest when you speak to her. Some thugs have been harassing her girls and she wants you to check it out. You can only complete this quest after dark when the Salamdra come out.
Patrol the streets looking for trouble. You’ll find on group of bandits harassing a working girl between the buildings at (#2), another group in the non-human district at (#3) and the last group around the market area (#4).
Once you’ve dealt with the bandits head back to Carmen for your reward. You can either take a cash reward of 200 Orens or have the eternal gratitude of the working girls who will show Geralt a good time every time you give them some flowers.