Make an equation. x will be the third number.
To find the average, you add up the numbers then divide the sum by the number of numbers you added up.
(8 + 9 + x) ÷ 3 = 7
Now solve. Add up like terms (8 + 9).
(17 + x) ÷ 3 = 7
Multiply both sides by 3.
17 + x = 21
Subtract both sides by 17.
x = 4
The third number is 4.