Remember you cannot take the square root of a negative value. So that means the argument must be greater than or equal to zero (i.e. the argument must be positive)
Set the inner expression greater than or equal to zero
Subtract 21 from both sides
Combine like terms on the right side
Divide both sides by 3 to isolate x
Divide
So that means x must be greater than or equal to -7
So here is the domain in interval notation: [-7,)
Notice if we graph , we get
notice how the graph never crosses the line . So this graphically verifies our answer.
and we can see that x must be greater than or equal to -7 in order to lie on the graph