（1）a^2-2(ab-b^2)-b^2
=a^2-2ab+2b^2-b^2
=a^2-2ab+b^2
(2)(x^2-y^2)-3(2x^2-3y^2)
=x^2-y^2-6x^2+9y^2
=x^2-6x^2-y^2+9y^2
=-5x^2+8y^2
(3)7a^2b-(-4a^2b+5ab^2)-2(2a^2b-3ab^2)
=7a^2b+4a^2b-5ab^2-4a^2b+6ab^2
=7a^2b+4a^2b-4a^2b-5ab^2+6ab^2
=7a^2b+ab^2