This is probably pretty easy, but not for me !
If you have a coordinate x,y,z in world space, how do yo find out what that coordinate would be in another coordinate system ?
In other words, you have a matrix3f and a vector3f, what would that vector be "seen" from another matrix ?
I know its a nob question but I'm a math nob 
Well what im trying to do is to figure out if a box hit the "backside" of another box or the "front side" !! Maybe there's a better way for that ?