x²-4xy+4y²-5（x-2y）=3
(x-2y)²-5(x-2y)=3
(x-2y)²-5(x-2y)+25/4=3+25/4
(x-2y-5/2)²=37/4
x-2y-5/2=±1/2√37
x-2y=5/2±1/2√37
∴2x-4y=5±√37