We require that the expression under the radical, the "radicand",
is not negative, but is 0 or something greater. 





The expression on the left has zeros 3 and -3, so we
mark these solid on a number line:

-------------@-----------------------@--------
-6  -5  -4  -3  -2  -1   0   1   2   3   4   5  

We choose a test value on the left of -3, say -4, and
substitute it in:







This is false so we do not shade to the left of -3
So we still have:

-------------@-----------------------@--------
-6  -5  -4  -3  -2  -1   0   1   2   3   4   5

We choose a test value between -3 and +3, say 0, and
substitute it in:







This is true so we shade the part of the number line between
-3 and +3 inclusive of -3 and 3 since the inequality is 
and not >.  So we have:

-------------@=======================@--------
-6  -5  -4  -3  -2  -1   0   1   2   3   4   5

We choose a test value on the right of +3, say +4, and
substitute it in:







This is false so we do not shade to the right of 3
So we still have:

-------------@=======================@--------
-6  -5  -4  -3  -2  -1   0   1   2   3   4   5

and the domain is [-3,3]

The integers in the domain are -3, -2, -1, 0, 1, 2, 3.

So the answer is 7, choice d.

Edwin