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