Im assuming what point your at in the game, as you dont say.
Im also assuming you blew the bridge with the tanks on it, im not sure what happens if you fire before they are, it seemed obvious to wait so ive never tried it.
Is it the russian tank in the park area your trying to take out???, if so look on either side of it above the tracks, toward the rear, there's a cylinder running down either side, look for the flashing red fuel filler cap on them and hit it, bye bye tank.