The initial idea here is familiar. Look for a mating threat for Black’s queen. Black’s bishop at c7 is attacking h2. Qe5 thus suggests itself, as it creates a mate threat against h2 and also attack the knight on d4. The knight is attacked once and defended once, so as we have seen it is a good target for a double attack. But we also have seen that it is important to pay attention to move order. Should Black play Qe5 and plan RxN on the next move? Or should he try RxN first and then play Qe5 after Black recaptures with RxR? This time you find the answer by asking what replies White could make after Black's initial Qe5. Don't automatically assume that White will move one of the pawns in front of his king; another possibility would be to play Nf3—shoring up the protection of h2 and also moving the target of the fork to safety. (You always want to ask whether the target of a fork would be able to jump to safety and block the check—especially when the target is a knight.) The preliminary exchange 1. …RxN; 2. RxR therefore is the way to go; then Black can play Qe5 and win the rook, as there is no way for White to move the rook and protect against mate at the same time.
The previous few problems show the importance of thinking about various move orders. Sometimes, as in this case, these patterns are best resolved with a preliminary exchange that puts a new piece on the target square; sometimes it's best to just play the mate threat and then perform an exchange afterwards—especially if a preliminary exchange would put a new piece on the target square that would be able to defend itself against a double attack. It all depends on how the exchanges would play out. The general lesson, applicable here and elsewhere: don't take move order for granted.