Explanation

Given:

Average of x + 3, x + 6, and x + 9 is 0

((x + 3) + (x + 6) + (x + 9)) / 3 = 0

Combine like terms:

(3x + 18) / 3 = 0

Multiply both sides by 3:

3x + 18 = 0

Subtract 18 from both sides:

3x = -18

Divide by 3:

x = -6