Explanation:

 In AP series, If the number of terms are odd, then middle number is the average of the series.

So Assume 'x+1' to be a number 7   => x+1 = 7 ;    x = 6

Hence 'x -1' will become 5

Since 'x+1' and 'x-1' are the averages; So Let's make the two series having these averages.

4  5  6  7   8  9  10      (Average is highlighted)

And

2  3  4  5  6  7  8  9     (Average is highlighted)

Let's identify the non repetitive number of these 2 series.

2  3  4   5  6  7  8  9  10

Required Average = Sum of the Above Terms/9 = 54/9 = 6   (Here 6 stands for 'x')