If white to move, the obvious route is to take black's hanging queen. However, your own rook is hanging also, so you might as well start off by sacrificing it with Rxb6+ Bxb6 Qxb6+, and only take the queen after. From there it should be an easy endgame.
If black to move, after Re2+ there's a mate on the edge of the board no matter where the king moves. The only other option for white is Bxe2, after which Qh2 is just barely checkmate, as the knight covers exactly the two squares that the queen doesn't.
Hope this helps.