#### Answer

$(3, +\infty)$

#### Work Step by Step

The denominator is not allowed to be zero as division of zero leads to an undefined expression.
This means that $x$ cannot be equal to $3$.
The radicand (expression inside a radical) of a square root cannot be negative as its root is an imaginary number.
Thus, the value of $x$ can be any real number that is greater than or equal to $3$.
The restrictions to the value of $x$ are:
(1) $x \ne 3$;
(2) $x \ge 3$
This means that the value of $x$ has to be greater than $3$.
Therefore, the domain of the given function is $(3, +\infty)$.