The domain of a function is the set of input or argument values for which the function is real and defined.
So, for the given function to be defined, we need to find the possible values for which the values of x makes the square root to be positive.
That is;
-9 -5x ≥ 0
Now, let's solve for x
Add 9 to both-side of the equation
-5x ≥ 9
Divide both-side by -5
x ≤ -9/5
Therefore, the domain of the function can be represented in interval notation as: ( - ∞ , -9/5]