Thanks for the query.
By examining the sequence, we can see that -
First 6 is added to 3 to make it 9 and then 4 is subtracted to make it 5.
Then, 7 is added to 5 to make it 12 and 5 is again subtracted to make it 7.
Then, 8 is being added to 7 to make it 15 and 6 is being subtracted to make it 9.
From the above, we can say that if a is being added to n then a - 2 is being subtracted from a + n.
So, if the number is 9 and 18 is the next number then 9 is being added to 9.
So, the next number will be 18 - 7 = 11.