"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)
