a2-b2=(a-b)(a+b)
(4-x)*(4+x)=-2
(a-4)*(a+4)=2-
(4-cd)*(4+cd)=-()2
(9b-4)*(9b+4)=*2-
(9b-4)*(9b+4)=(*)2-
(15-m)*(15+m)=-2
(z-15)*(z+15)=2-
(15-cd)*(cd+15)=-()2
(4b-15)*(4b+15)=*2-
(4b-15)*(15+4b)=(*)2-
(9ab-zx)*(zx+9ab)=*()2-()2
(9z+a)*(a-9z)=2-*2
(mlu+9k)*(9k-mlu)=*2-()2
(15bc3-9x)*(9x+15bc3)=*b2c6-*2
(15bc3-9x)*(9x+15bc3)=(*bc3)2-*2