Mean : The "mean" is the "average" , for which add up all the numbers and then divide by the number of numbers.

The given set of data; 98, 121, 104, 113, 107, x
Sum of all data : 98 + 121 + 104 + 113 + 107 + x = 543 + x
Total numbers of data : 6
Mean of the data : 116
Substitute the value in the expression of mean;

Simplify the expression for x;

So, the number is 153
Answer : x = 153