Part A:
You may choose the two lines connecting the origin and points A and B, and choose the portion of the space between them.
The line between the origin and A is
[tex]y = 3x[/tex]
We want everything below this line (line included), so the first inequality is
[tex]y \leq 3x[/tex]
The line between the origin and B is
[tex]y = \dfrac{1}{3}x[/tex]
We want everything above this line (line included), so the second inequality is
[tex]y \geq \dfrac{1}{3}x[/tex]
Create a system with these two inequalities and you'll have an area including only points A and B
Part B:
To verify the solutions, we can plug the coordinates of A and B in this system and check that we get something true: the coordinates of point A are (1,3), while the coordinates of point B are (3,1). The system becomes:
[tex]A:\begin{cases}3 \leq 3\cdot 1\\3 \geq \frac{1}{3}\cdot 1\end{cases},\quad B:\begin{cases}1 \leq 3\cdot 3\\1 \geq \frac{1}{3}\cdot 3\end{cases}[/tex]
Which means
[tex]A:\begin{cases}3 \leq 3\\3 \geq \frac{1}{3}\end{cases},\quad B:\begin{cases}1 \leq 9\\1 \geq 1\end{cases}[/tex]
And these are all true. So, the system is satisfied, which means that the points belong to the shaded area.
Part C
If you draw the line, you'll see that the only points that lay below the line are B and C. In fact, if we plug the coordinates we have
[tex]B:\ 1 <3\cdot 3 - 6 \iff 1 < 3,\quad C:\ -3 < 3\cdot 3 - 6 \iff -3 < 3[/tex]
And this are both true. You can check the coordinates of all other points, and see that they won't satisfy the inequality y<3x-6