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