已知A=5x^2y-3xy^2+4xy,B=7xy^2-2xy+x^2y.
（1）求A-2B的值
A-2B
=5x^2y-3xy^2+4xy-2*(7xy^2-2xy+x^2y)
=5x^2y-3xy^2+4xy-14xy^2+4xy-2x^2y
=3x^2y-17xy^2+8xy
（2）若A+B+2C=0,求C-A的值.
因为A+B+2C=0,所以A+B=-2C
5x^2y-3xy^2+4xy+7xy^2-2xy+x^2y=-2C
6x^2y+4xy^2+2xy=-2C
C=-3x^2y-2xy^2-xy
C-A=-8x^2y-5xy^2+3xy