-(3a的2次方-b) (b+3a的2次方)
=(b-3a的2次方) (b+3a的2次方)
=b^2-(3a^2)^2
=b^2-9a^4
(7m+2n) （-7m+2n)
=（2n+7m)(2n-7m)
=(2n)^2-(7m)^2
=4n^2-49m^2