9)(x-y)^2-x(x-2y)=x^2-2xy+y^2-x^2+2xy=y^2
10)a(a+2b)-(a+b)^2=a^2+2ab-a^2-2ab-b^2=-b^2
11)(a-3)(a-7)-2a(3a-5)=a^2-10a+21-6a^2+10a=-5a^2
12)(x-2)(x+4)-2x(1+x)=x^2+2x-8-2x-2x^2=-x^2-8
13)2c(3c+4)-3c(2c+1)=6c^2+8c-6c^2-3c=5c
14)3a(2a-1)-2a(4+3a)=6a^2-3a-8a-6a^2=-11a