Jonnie wants you to acquire a rocket launcher from the docks and bring it back to him so he can take out some suspicious people who have been driving past his bar recently.
Firepower
Make your way to the docks where you'll find the weapon waiting for you. Once you pick it up, you'll be attacked by three guys on foot. Upon killing them, two or three more will come at you, alongside a vehicle. Keep shooting them until they're all dead, and/or blown up. Don't use the RPG though.
Once they're all wasted, head back to the bar with the Rocket Launcher.