I'm not sure whether I understand you correctly (I am not genius, so let me know if I am wrong). The problem is that you have world matrix for the board and world matrix for the tank and you need to obtain the position of the tank in world and its rotation, right?

You can obtain world position from the matrix by inversing it and then extracting x, y, and z from the last column. The same applies for rotation since basically you want to rotate the world in reverse to tank rotation, so you see everything from the front of the tank.

In Direct3D there is D3DXMatrixDecompose function, which can do the trick. Once I saw a link to a good article about how to do it manually (or extracting rotations in different order) on GameDev forums, but don't remember the link. A quick search came to this page, which may be helpful.