First way: Work from the outside in. Distribute the 3 to get 3x + 3y(x - 2) + 12y. Then distribute the 3y to get 3x + 6xy - 6y + 12y. Finally, combine like terms to get 3x + 6xy + 6y. Second way: Work from the inside out. Distribute the y to get 3[x + xy - 2y + 4y]. Combine like terms to get 3[x + xy + 2y] and finally, use the distributive property once again to get 3x + 3xy + 6y. We get the same answer either way. Kind of like when you first try nicely asking your parents if they can buy you an iPhone, and then you try demanding it. Either way, you ain't getting an iPhone. | 
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