Answer: The missing number should be 23.
Step-by-step explanation: Here, we can see that the difference between the consecutive terms are not equal, so, we have to apply some alternative technique to find the answer.
We have
38-26=12 and 26-21=5.
Also, 12-5=7
Therefore, 5-7=-2, and when we subtract -2 from 21, the result is 23.
Thus, the missing number is 23.