White’s knight has forked your queen and f8 rook. What to do? Don’t be quick to think defensively; calmly consider your own offensive options. Start by observing the action on White’s back rank: his king is stuck there, almost enabling Black to mate with RxR—but White’s queen guards his rook on f1. This makes the queen vulnerable; its ability to fend off an attack against itself is limited. So Black looks for a way to attack it. Better still, he looks for a way to attack it and attack something else at the same time. Noticing that White’s knight on e6 is loose, Black plays Qd7, a fork of White’s queen and knight. Ordinarily a queen makes a poor target for a queen fork, but this is one of those cases where if the attacked queen strikes back, mate results. (We saw a bunch of these positions in our studies of double attacks by the queen.)
White’s preference would be to move the queen to a square where it is safe and can guard the knight and the mating square f1, so he plays Qc4. It's important to ask whether such a substitute square exists, and to see this one—but then important also to notice that by guarding those two points the queen is left overworked. Black therefore goes ahead with RxR+, requiring the reply QxR; and now Black has QxN, winning a piece.
As you might expect when you start with an attack of this sort, there are other ways the position can play out, too, and you want to see them. After Black’s initial Qd7, White can play RxR; and then, after Black plays QxQ, White has NxRf8 (winning the exchange after Black recaptures with BxNf8). This way Black ends up winning a queen and a knight for two rooks.