White to move. Is this a win or draw? How should white proceed?
a) 1...Ka5, 2. Be4! Ka4, 3. Bc6+ Ka5, 4. Ra3+ Ra4, 5. Rxa4#
b) 1...Rf5, 2. Re2! (threatening 3. Ra2) Rxd5+, 3. Kxd5 and White wins
I can't immediately tell you what doesn't win here, but the win from here pretty simple as far as this ending goes. 1.Re3 is the quickest win available since the black rook is on f4 in this position and the black king is on a white square- the color of the bishop:
1.Re3! Rf5 (the longest defense)
If black tries a move like 1...Rg4, white just checks from c6 with the bishop and mates in in 4 moves total. This applies to other rook moves, too, except for 1...Rf6 in which case white can win by just forcing black to lose the exchange at c6, but can win even quicker with 2.Be4 forcing the rook away from c6 so that the mate can be delivered more quickly. Continuing:
This threatens 3.Ra2# There are no king moves to save black, so he has to concede the exchange anyway, or divert the bishop to f3 by sacrificing the rook.