6÷2(2+1) is not a valid mathematical expression.

Although we usually write expressions as (e.g.) 2(x+4), this is the lazy & taken for granted way of writing it. The correct form is 2*(x+4). 

There must be an operator between the number 2 and the parenthesis. Without it, it is both incomplete and would also lead to confusion. 

For that example, wolfram alpha computed that as 9. Which means it corrects the expression to have a multiplication operator between the 2 and (. 

https://www.wolframalpha.com/input/?i=6%C3%B72(2%2B1)