3+3+3+4+4+5+6+6+7+9+9=59
59/11=5.36
so your mean is 5.36

Hope you could get an idea from here.
Doubt clarification- use comment section.
Answer:
-6 = x / 3 -Multiply both sides by 3
-6 * 3 = x / 3 * 3
-18 = x
Now, you can check your answer:
-6 = -18 / 3
-6 = -6
Hope this helps!
We can write out a formula in order to solve this. Since sum represents addition, if x represents the unknown number, we can write the formula: x+51=-21. Then, to solve for x, you would subtract 51 from both sides to get x=-72 which is the final answer.