White’s knight is on a dark square; so are Black’s bishop and rook. You see a knight fork at c6. But it won’t work because Black can respond by moving his rook to e8, both taking it out of harm’s way and using it to protect the bishop. As we know, this is a common difficulty when forking two pieces neither of which are the king; one often can move to protect the other. Can you improve the target by capturing one of the pieces and forcing an exchange? Yes, with RxB+, to which Black replies KxR. Then comes Nc6+, and now White wins the rook; moving the rook to protect the other piece doesn’t work anymore for Black because now the other piece is a king.