Answer
The domain is
$\mathcal{D}=\left\{(x,y)|y\leq\frac{1}{3}x\right\}$ and it is shown in the figure below
Work Step by Step
The argument of every even order root has to be positive so we need that $x-3y\ge0$ which gives $3y\le x$ and finally $y\le\frac{1}{3}x.$ We can now write for the domain
$$\mathcal{D}=\left\{(x,y)|y\le\frac{x}{3}\right\}.$$
Geometrically, this is the region of $xy$ plane under the line $y=\frac{1}{3}x$ (including this line). It is shown on the graph below
