Would someone mind explaining this?

Answer:
3?
Step-by-step explanation:
i think that because it's multiplying by 3.
3*3= 9
5*3= 15
7*3= 21
9*3= 27
This is the only way i could think of helping in some sort of way! I'm sorry if it's wrong! (´。_。`)
Have a great day! (´。_。`) / (ノ◕ヮ◕)ノ*:・゚✧
( ゚д゚)つ Bye