由已知得：x=2y/3,代入欲求式子,得
(7x^2-y^2)÷(x^2-2xy+3y^2)
=[7*(2y/3)^2-y^2]÷[(2y/3)^2-2*2y/3*y+3y^2]
=(28y^2/9-y^2)÷(4y^2/9-4y^2/3+3y^2)
=19y^2/9÷19y^2/9
=1