1)(-c+d)(c+d)=-(c-d)(c+d)=-(c^2-d^2)=-c^2+d^2
2)(-a+b)(b-a)=(b-a)(b-a)=(b-a)^2=b^2-2ab+a^2=a^2-2ab+b^2
3)(-x-y)(x-y)=-(x+y)(x-y)=-(x^2-y^2)=-x^2+y^2
4)(a+b)(-a-b)=-(a+b)(a+b)=-(a+b)^2=-(a^2+2ab+b^2)=-a^2-2ab-b^2
5)(x-y)(y-x)=-(x-y)(x-y)=-(x-y)^2=-(x^2-2xy+y^2)=-x^2+2xy-y^2
6)(-a-b)(-a-b)=-(a+b)*(-(a+b))=(a+b)^2=a^2+2ab+b^2