Answer
$$global~max: 2,\ \ \ \ \ \ global~min: -3$$
Work Step by Step
Given $$f(x, y)=2 x-y, \quad 0 \leq x \leq 1, \quad 0 \leq y \leq 3$$
The maximum of $x$ is $1$ and $y$ is $3$; the minimum of $x$ is $0$ and $y$ is $0$. We see that the maximum of $2x-y $ is $2-0$ and the minimum is $0-3$. Hence, the global maximum of $f$ on the given set is $$f (1, 0) = 2-0=2$$ and the global minimum is $$f (0,3) = 0-3 =-3$$
