（3x-y)的平方-（2x+y)的平方-5x(x-y)
=(3x-y+2x+y)(3x-y-2x-y)-5x(x-y)
=5x(x-2y)-5x(x-y)
=5x(x-2y-x+y)
=5x*(-y)
=-5xy
=-5*2*1
=-10