Both (7bâ‹…3)+(7bâ‹…c) and 21b+7bc are correct choices.
Let's take a look at the choices and see if we can manipulate 7b(3+c) to make it match.
14bc
* I can't think of any way to change 7b(3+c) to make this equation. So bad choice.
(7bâ‹…3)+(7bâ‹…c)
* This looks like the multiplication of 7b was distributed to the 2 terms of the addition. That's a legal manipulation and this is a good choice.
â’4b+7c
* I can't think of any way to change 7b(3+c) to make this equation. So bad choice.
21b+7bc
* This looks like the multiplication of 7b was distributed getting (7bâ‹…3)+(7bâ‹…c) and then the multiplications were either done, or made implicit. These are legal manipulations and this is a good choice as well.
7b+14bc
* I can't think of any way to change 7b(3+c) to make this equation. So bad choice.
