OKay, this is an anti-puzzle. White to AVOID checkmating black !!!
Three possible solutions:
1. Offer a draw.
2. If Black is stupid enough to decline the offer, let your time run out. The arbiter has to declare the game a draw, since ALL moves are forced and lead to mate for white (which means, Black has no winning chances).
3. Resign (fastest, but costliest).
thank for the correct answer. who would have guessed that?
You're right Adriandmen the answer was to go via b7 with a knight in between