Looks to me like your answer is the same as the one in the book, with the exception that you have a term of , by which I assume you mean $7|\vec a| \vec b|$, whereas the book's answer is $7 \vec a \cdot \vec b $. The reason the book has the dot product is that $ \vec a $ and $ \vec b$ are not necessarily orthogonal, the way perhaps other examples in your book may be.