Explanation

Given that the sum of 3, 7, and x is 18:

3 + 7 + x = 18

Combine constants:

10 + x = 18

Subtract 10 from both sides:

x = 8

Now, find the average (arithmetic mean) of 3, 7, and x:

Average = (3 + 7 + x) / 3

= (3 + 7 + 8) / 3

= 18 / 3

= 6