Answer:
C. 3n+9>21
Explanation:
Ignore the 9 for a second and think about the phrase "3 times a number", if we say n is the number, then 3n is 3 times the number n.
Now we need something that is 9 more than this 3n, that means you add 9 to this, so you have 3n+9.
Finally, this 3n+9 is greater than 21, so we have 3n+9>21.