a2-b2=(a-b)(a+b)
(4-x)*(4+x)=-2
(a-4)*(a+4)=2-
(4-cd)*(4+cd)=-()2
(7b-4)*(7b+4)=*2-
(7b-4)*(7b+4)=(*)2-
(9-m)*(9+m)=-2
(z-9)*(z+9)=2-
(9-cd)*(cd+9)=-()2
(4b-9)*(4b+9)=*2-
(4b-9)*(9+4b)=(*)2-
(7ab-zx)*(zx+7ab)=*()2-()2
(7z+a)*(a-7z)=2-*2
(mlu+7k)*(7k-mlu)=*2-()2
(9bc3-7x)*(7x+9bc3)=*b2c6-*2
(9bc3-7x)*(7x+9bc3)=(*bc3)2-*2