Вариант 1: (m-2n)(m+2n-1)

(m-2n)(m+2n-1) = m(m+2n-1) - 2n(m+2n-1) = m^2 + 2mn - m - 2mn - 4n^2 + 2n = m^2 - m - 4n^2 + 2n