Ok, so I'm self-teaching myself Algebra over the summer so that's one less math class I'll have to worry about and I have a question about 2 algebra problems. I'm trying to find out how exactly the author of the book I'm using got the answer to these 2 questions and it would help a lot if you could explain why the answers are the answers. I believe monomial's and polynomial's using the distributive law.

The first 2x^2y(x^2-xy+3y) and the answer in the book was 2x^4y - 2x^3y^2+6x^2y^2

The second a/2 (2a-4+3b) and the answer was a^2-2a+3ab/2