_________-3(x^2-2b)=2x^2-b

（1）
__5x²-7b___-3(x^2-2b)=2x^2-b
2x^2-b+3(x^2-2b)=2x²-b+3x²-6b=5x²-7b
（2）
m^4-2m^2n^2-2m^2+2n^2+n^4=m^4-2m^2n^2-( 2m²-2n² )+n^4
（3）
多项式5x^2y-15x^2y^2与12xy^2+23x^2y^2-x^2的差是5x^2y-38x^2y^2-12xy^2+x^2
5x^2y-15x^2y^2-(12xy^2+23x^2y^2-x^2)
=5x^2y-15x^2y^2-12xy^2-23x^2y^2+x^2
=5x^2y-38x^2y^2-12xy^2+x^2
(4)
若ab=-2,a-b=1/4,则（ab-4a）+a-3b的值为
(a+b)²=(a-b)²+4ab=1/16-8

( 5x^-7b ) -3(x^2-2b)=2x^2-b

m^4-2m^2n^2-2m^2+2n^2+n^4=m^4-2m^2n^2-( 2m^2-2n^2 )+n^4
5x^2y-15x^2y^2-(12xy^2+23x^2y^2-x^2)=5x^2y-12xy^2-28x^2y^2+x^2

ab=-2
a-b=1/4
(ab-4a)+a-3b=