7(n^2-2)-4(3+n)(n-3)= (7n^2-14)- 4(n^2-9)= 7n^2-14-4n^2+36= 3n^2+22
a: ((m-2n)*(m-2n)):((m-2n)*(m+2n)=(m-2n):(m+2n)