White’s rook is under attack by Black’s king. He has to move it. Before retreating, though, think about ways of taking the offensive. What does White himself threaten? His king attacks Black’s rook in mirror-image fashion (the position basically is symmetrical, of course). The rook is guarded by the knight, so White simply removes the knight from the board: RxN+. This forfeits the exchange, but for the sake of loosening Black’s rook. However Black recaptures, White has KxR next move and wins a piece.