Answer
9
Work Step by Step
Since the mean of the seven numbers is 0, we know that the sum of all seven numbers divided by seven is 0.
So, we know that
$\frac{sum-of-all-seven-numbers}{7} = 0$
The sum of all seven numbers is composed of the sum of three of the numbers (which is given to us in the problem) plus the sum of the remaining four numbers (which is what we are trying to find).
$\frac{(sum-of-three-numbers )+ (sum-of-remaining-four-numbers) }{7} = 0$
We know the sum of the three numbers is -9.
$\frac{-9 + (sum-of-remaining-four-numbers) }{7} = 0$
So all we have to do is solve for what we want. First multiply each side by 7:
$-9 + (sum-of-remaining-four-numbers) = 0$
And solve for the answer by adding 9 to both sides:
$sum-of-remaining-four-numbers = 9$
So, the answer is 9.