I think questions like this one are always really hard to understand and I would like another mathematician to check my answer.
Use my pic, which is near enough to the same.
Think about a horizontal line through the graph.
At the top it will pass through the graph twice so g(c)=2
Here is a pic
I have drawn six horizonal lines.
For the top (purple - 1st ) one f(x)=-2
this crosses the graph f(x) at 2 different x values so g(-2)=2
the next horizontal line (black 2nd ) palsses through 4 points so g(-2.8)=4
the 3rd horizontal line (red ) palsses through 6 points so g(-3.4)=6
the 4th horizontal line (blue ) palsses through 5 points so g(-4)=5
the 5th horizontal line (green) palsses through 4 points so g(-5)=4
the bottom horizontal line (purple ) palsses through 2 points so g(-6.7)=2
So g(c) can be 2, 4, 5, or 6
The average of the distict values of g(c) = (2+4+5+6)/4 = 17/4 = 4.25
I am pretty sure that is correct.
Here is the graph I used https://www.desmos.com/calculator/ft9bo78w4l
You do not need it though.