Answer
Domain: $x\leq 4$
Range: $y\geq 0$
Work Step by Step
Given \begin{equation}
f(x)=\sqrt{4-x}.
\end{equation} The radicand of a radical function must be positive. This means that we must have $$4-x\geq 0\implies 4\geq x\implies x\leq 4.$$ Also, we see that the function's value must satisfy $y\geq 0$ because $\sqrt{4-x}\geq 0$.
We got: \begin{equation}
\begin{aligned}
\textbf{Domain :}\quad & x\leq 4\\
\textbf{Range :}\quad & y\geq 0\\\\
\end{aligned}
\end{equation}
