cut the Gordian knot
To solve a very challenging or daunting problem decisively. The phrase likely alludes to Gordius, the king of Phrygia, who tied a knot that an oracle proclaimed would only be cut by the future ruler of Asia. Alexander the Great allegedly cut the Gordian knot in one blow. A: "Wait, Matt already solved that impossible equation?" B: "Yes! I have no idea how he did it, but he sure cut the Gordian knot."