Later in the game you will encounter a courtyard. Take out all the snipers and kill the remaining enemies. After you kill all of them, a red truck with a 50 cal on it will approach from the road ahead. After taking out the .50 cal, go across the street to the library and into the computer store. Looking through the window, throw a smoke grenade at the tank. The tank now cannot see you. Get in position and take out your M203 or grenades and start firing. It takes about four grenades; do not wait too long or the smoke will wear off, and the enemy will open fire.