Trig-Type Help

1

I’m making a game (Viewable here: http://www.geocities.com/djorkon/BattleProto.html ) and I need help. When you get in the one vehicle on the screen and go towards your enemies, how can I make it so that when a bullet hits the vehicle it deflects off in the same way a billiards ball in a frictionless invironment would deflect off a wall? I want to know how to do this with triginometry, because I’m not using xSpeed and ySpeed. I am currently just reversing the bullet’s rotation.