Answer:
21 + 7b
Explanation:
We have the expression 7(3 + b) and are being asked the answer.
Our expression has a number out of a certain parenthesis, when this occurs, this means that we must distribute the number outside the parenthesis to the numbers inside. Do as following :
7(3 + b)
(7(3) + 7(b))
21 + 7b
(I am unsure on whether you forgot the "= 0" at the end, if you did please tell me in the comment section and I will change my answer.)