(3x^2-4y^2)+[-(x^2-2xy-y^2)]-[-(3x^2-2xy-y^2)]
=3x^2-4y^2-(x^2-2xy-y^2)+(3x^2-2xy-y^2)
=3x^2-4y^2-x^2+2xy+y^2+3x^2-2xy-y^2
=5x^2-4y^2