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-
(15-m)*(15+m)=-2
(z-15)*(z+15)=2-
(15-cd)*(cd+15)=-()2
(9b-15)*(9b+15)=*2-
(9b-15)*(15+9b)=(*)2-
(7ab-zx)*(zx+7ab)=*()2-()2
(7z+a)*(a-7z)=2-*2
(mlu+7k)*(7k-mlu)=*2-()2
(15bc3-7x)*(7x+15bc3)=*b2c6-*2
(15bc3-7x)*(7x+15bc3)=(*bc3)2-*2