4. Find the greatest and least values of the expression $x-2 y$, if $(x ; y)$ are related by the relation $\sqrt{x-2}+\sqrt{y-3}=3$. For which $(x ; y)$ are they achieved?
Answer: $(x-2 y)_{\max }=5$ when $x=11, y=3,(x-2 y)_{\min }=-22$ when $x=2, y=12$.