"The sum of a number and 7"
Sum implies addition.
We would write this part as n + 7.
Then we would write the "9 times" part in like this:
9 · (n + 7)
We can't write just 9 · n + 7 without the parentheses, because then we would be multiplying just 9 and n instead of 9 and the sum of n and 7. (order of operations says we must do multiplication first)
