This time the key to the mate threat is to examine Black’s king and its range of motion or lack thereof. It cannot move at all. A king so trapped is terribly vulnerable, so you should look for a check you can give that might snuff it out. White has two with his e5 knight. Ng6+ loses the knight to Black’s f7 pawn. The other, Nxf7+, is more interesting because all that prevents it from being mate is the protection provided to f7 by Black’s queen. We know that when a queen stops you from mating, its own freedom of action is limited and it becomes a potential target itself. How might you attack it? Ask about the availability of each of your pieces for the purpose; see that you can do it with your dark-squared bishop: Bd6. The bishop is loose there, but it doesn’t matter. If Black takes it, White mates. The only question is whether Black can move his queen to a safe square from which it still can protect f7. He cannot. It would have to be another square on the seventh rank, and none are both safe and available. What Black can play is Qe7; after White plays BxQ, this at least allows the recapture RxB—and now the rook is on the seventh rank, able to keep protecting f7. But the loss for Black still is disastrous.