Well you didnt list a pattern so if you need to learn a pattern how about maybe..
3 6 9 12 15
Adds 3 every time
9, 4.5, 2.25, 1.125
Divides 2 everytime
Just do the opposite of whatever mathematic system youre using to find the pattern.
Like maybe add 4.5+4.5 and it would be 9. Like
It would turn into a whole new equation
Find for 9.
or find for 6.