Talk to Mira with a stupid character and at least ST 7; Merk will point you in her direction if you talk to him. She'll tell you to climb a rope over the city wall. If it's nighttime when you enter downtown, Jack will be in the police station. Wait until dawn, and he'll walk over to the power station and threaten to blow himself up, which is the best opportunity to waste him without annoying anyone; you can actually let him blow himself up with the same result. When he's dead, head over to the bazaar and talk to Mira for your reward.
This quest pops up when Jack dies even if it wasn't active before, so Mira's reward can be earned despite not getting the extra XP for killing Jack. If Jack was allowed to suicide bomb she says she'll give a rad-a-waster, but gives no booze like normally if one was bought. If Jack is killed, however, she gives a HK P90c.
For a normally non-stupid character, the best time to get this quest would be after talking Jack out of blowing himself up. Use Mentats if Intelligence is below 7, Psycho and/or Buffout to get the quest, then go kill Jack silently in his cell for the 2000 XP. IN must go back below 4 to collect the reward.
If fought Lenny in the ring, Mira's dialogue will be different won't start the quest, and if already gotten the quest Mira's reward can't be gained (whether 'stupid' or not).