a) (5m + 7n)(2n + 4m)

Раскрываем скобки: (5m)(2n) + (5m)(4m) + (7n)(2n) + (7n)(4m) = 10mn + 20m^2 + 14n^2 + 28mn = 20m^2 + 10mn + 28mn + 14n^2 = 20m^2 + 38mn + 14n^2.