I need to factor and I do not understand this equation, please help

3a^2(a-b)-6a(a-b)+21(a-b)

1)3(a-b)(a^2-2a+7)
2)(3a-b)(a^2-2a+7)
3)(a-b)(3a^2-6a+21)
4)3(a-b)(a^2-6a+21)