m² - mn + ma - na= m(m - n) + a(m - n)= (m - n)(m + a) )(2a - b - c) = (2/3)a² - (1/3)ab - (1/3)ac= [(2/3)a² - (1/3)ab - (1/3)ac]/(2a - b - c)= [(a/3)(2a - b - c)]/(2a - b - c)= a/3