7x^2 - 7y^24m^3 - 4mn^25x^2 - 20y^2a^3b - ab^3 = 7(x^2 - y^24m^3) - 4mn^2(5x^2 + 5y^2a^3) - ab^3 = 7(x - y^2m)(x + y^2m) - 20mn^2(x^2 + y^2a^3) - ab^3 = 7(x - y^2m)(x + y^2m) - 20mn^2(x + y)(x - ya^2) - ab^3
7x^2 - 7y^24m^3 - 4mn^25x^2 - 20y^2a^3b - ab^3
= 7(x^2 - y^24m^3) - 4mn^2(5x^2 + 5y^2a^3) - ab^3
= 7(x - y^2m)(x + y^2m) - 20mn^2(x^2 + y^2a^3) - ab^3
= 7(x - y^2m)(x + y^2m) - 20mn^2(x + y)(x - ya^2) - ab^3