a2-b2=(a-b)(a+b)
(7-x)*(7+x)=-2
(a-7)*(a+7)=2-
(7-cd)*(7+cd)=-()2
(7b-7)*(7b+7)=*2-
(7b-7)*(7b+7)=(*)2-
(13-m)*(13+m)=-2
(z-13)*(z+13)=2-
(13-cd)*(cd+13)=-()2
(7b-13)*(7b+13)=*2-
(7b-13)*(13+7b)=(*)2-
(7ab-zx)*(zx+7ab)=*()2-()2
(7z+a)*(a-7z)=2-*2
(mlu+7k)*(7k-mlu)=*2-()2
(13bc3-7x)*(7x+13bc3)=*b2c6-*2
(13bc3-7x)*(7x+13bc3)=(*bc3)2-*2