a2-b2=(a-b)(a+b)
(9-x)*(9+x)=-2
(a-9)*(a+9)=2-
(9-cd)*(9+cd)=-()2
(7b-9)*(7b+9)=*2-
(7b-9)*(7b+9)=(*)2-
(2-m)*(2+m)=-2
(z-2)*(z+2)=2-
(2-cd)*(cd+2)=-()2
(9b-2)*(9b+2)=*2-
(9b-2)*(2+9b)=(*)2-
(7ab-zx)*(zx+7ab)=*()2-()2
(7z+a)*(a-7z)=2-*2
(mlu+7k)*(7k-mlu)=*2-()2
(2bc3-7x)*(7x+2bc3)=*b2c6-*2
(2bc3-7x)*(7x+2bc3)=(*bc3)2-*2