Buy a fast Rank C car and race the street gang. This race will be tough if you are using the free 600 Coupe S you got earlier.
You get a call and need to move around to triangulate it (head towards Wah Fi Storage). The scanning minigame will require you to locate the correct green person pin-marker that is common across all four "pings" - wait for the ping and see which marker consistently appears in the same spot across different pings and scan it.
If the call is lost (when the woman drops the call), you fail and need to try again. Once you have the pin and scan the marker, race there (hence the reason for the fast car) before the victim dies.
Check out the crash site on the beach, get a cutscene (where the woman tells Wei that Hotshot killed Ace) and photograph the car while heading back up to the road. Call Teng, then Hotshot, and meet him at Victoria Peak with the fastest (Rank A) car (there's an Etalon at the parking lot at the crash site). Check the Speed Chart for the best one. Follow him to Teng's trap, disable his car, escape the cops, and you're done.