Can enyone tell me what I must to do in Virus House.

you shoot a bazooka at the window above the flag.

Do you mean the actual mission, or the location in the mission. Well, in the location, you just have to kill all German forces. Your allies stay outside. First, use any objects as a cover, because there are so many German troops in there. Peek around behind walls, crates etc. using the ADS aiming. Hold L1 and move your left analog stick to peek around, and shoot. You'll have to repeat this, grenades help a lot. And use your adrenaline powers whenever you can. When you meet Schrader, there's a lot of enemy troops coming for you. Use the crates in the silo area as a cover. Bazooka is helpful in this section too. (if you still have some ammo for it) Kill the Germans and finish Von Schrader, using cover, and aiming for the head. Then climb upstairs, and stop the launch. The door will open when you have killed Schrader. You'll have to throw grenades in the room above you, to kill Schrader, and shoot him if he appears in the window, trying to shoot you.