Answer:
+ 4
Explanation:
The pattern of the expression is +4 as you see it goes 5 to 9
5 + 4=9
And if i'm correct the number after 9 will be 13 and it is
9 + 4=13
next it will be a 17 because 13 + 4=17
The next following numbers of the pattern will be 21 25 29 33 37 41 45
Hope it helps have a great day:)