Answer
$$D=\{(x,y)\in R^2: x\geq y\}.$$
Work Step by Step
Given $$h(x,y)=\sqrt{x-y}.$$ Since there is no square roots for negative values, then the function $h$ is defined only whenever $x-y\geq0$ that is the domain is $$D=\{(x,y)\in R^2: x\geq y\}.$$
