Study what forces White has trained on the Black king’s position: his knight on g5; his queen on c2; his knight on d5; and his bishop on b3. The sight of that bishop aimed down the diagonal should cause you to think about possible pins; it has the potential to pin the pawn on f7, which would leave the g6 pawn loose, which the queen then could take with decisive results. How to create the pin? By vacating the White knight from the diagonal forcefully, leaving Black no time to escape. White thus plays Nd5xN+, forking (of course) Black’s king and queen. If Black wants to save his queen he replies BxN. But then the work is done; the exchange has cleared the way for a pin that permits White to play Qxg6+ with impunity. And since Black has had to move his bishop, White's move is a check. The Black king goes to h8, after which White mates with Qh7—this time with cover from its knight. (In the alternative, Black might reply to Qxg6+ by retreating his bishop to g7. White’s queen then delivers mate from h7.)