(m-2n-3)²
=[(m-2n)-3]²
=(m-2n)²-6(m-2n)+9
=m²-4mn+4n²-6m+12n+9
(2x-3)²(-2x-3)²
=[(-3+2x)(-3-2x)]²
=[(-3)²-(2x)²]²
=(9-4x²)²
=81-72x²+16x^4