This is a very simple task, especially since you've eliminated all of the nearby soldiers. You will have to go back to the previous street. An allied gunship will land here in a few seconds (#1). Get close to it and press the X key in order to enter the equipment screen (#2). You should replenish your ammo. If you have a sniper rifle with you, exchange it to a standard machine gun. Furthermore, you might consider taking the rocket launcher with you. CHECKPOINT 5.