.Qxg4 is a blunder for after ..Nxg4 .Bxe7 Bxe7 you have achieved nothing while .Bxf6 wins a piece. The difference between the two candidate moves is precisely what this puzzle intends to teach. And it is apparently just the lesson you need
Interestingly, black can save himself after .Bxf6 if you first remove Pf2 from the diagram. Can you see how? That's an advanced lesson.
In this puzzle problem, I played Qxg4 and after Nxg4, then Bxe7, which must be a true solution. Unfortunately, chess.com determined that Qxg4 is an incorrect solution, and the actual solution should be Bxf6, followed by Bxd1 for queen exchanges. I doubt that there is only one solution in this case, there must be absolute two possible solutions, right? Do anybody agree with my opinion? If not, please comment here below with any general ideas or explanations.