Playboy X informs Niko that he is catching flak on the streets because some punk is accusing him of being a snitch. Playboy wants Niko to take a picture of the punk so X can confirm that that's him. Use Playboy's Patriot to get to the basketball courts on Exeter Avenue between X and W nearby. After the cutscene, jog over to the hole in the fence. Open the new cell phone and go to the Camera option.
Take a picture of the nearby goons. Send it to Playboy so he can determine which one is the punk Niko is targeting. Playboy may call you to detail which one is the target or send a text message telling you to take a clearer shot. Follow his instructions; if you receive a text message, retake the picture. If you receive a call, take any weapon and end the punk. He will begin to run if he is not killed quickly. If it helps, kill his goons as well. After the mission, any remaining goons will open fire on you.
Alternatively you can just kill everybody, on the assumption that one of them must be Marlon.
There is also an alternative and less direct route in killing Marlon. After you take the picture and Playboy points out Marlon, head over to the nearest Pay n' Spray. There should be a ladder in an alley to the left of the Pay n' Spray. Climb it to find another ladder that will take you higher up the building. Climb one more ladder to have a perfect sniping view of Marlon's head. From this position, you will draw no attention to yourself until you have sniped Marlon. Just look out for Marlon's associate that starts pointing an Uzi around after you snipe Marlon because if you wait in your sniping position for too long, he will find you and start firing. To avoid this, it is best to snipe the associate with the Uzi as well after sniping Marlon. Either way, it's mission complete after killing Marlon.