What is the missing value in x15=6 ?

The equation \( x_{15} = 6 \) indicates that the 15th term (or value) in a sequence or set represented by \( x \) is equal to 6. Without additional context or information about the pattern or sequence, it's difficult to determine what "the missing value" refers to.

If you meant to ask for a value preceding or succeeding \( x_{15} \) in a specific sequence, please provide more details about the sequence or the context in which you are asking about the missing value.