1.(2a+3b)(3a-4b)=6a^2+ab-12b^2
2.(2x-3y)(3y-2x)-(4y-3x)(3x-4y)
=-(2x-3y)^2+(3x-4y)^2
=(3x-4y+2x-3y)*(3x-4y-2x+3y)
=(5x-7y)*(x-y)
=5x^2-12xy+7y^2
3.(x+3y)(x-2y)-2(x-y)^2
=x^2+xy-6y^2-2x^2-2y^2+4xy
=-x^2-8y^2+5xy
4.（x-5）（x+5）-（x-1）^2=（3x+5）（x-1）-3x(x+1)
x^2-25-x^2+2x-1=3x^2+2x-5-3x^2-3x
2x-26=-x-5
3x=21
x=7