It's the value of f(x) when x tends to 0. You just have to read the graph.
means that you have to consider , and , that is to say the limit of f(x) if x comes from the right towards 3.
is the same, but with , from the left.
Yup, because limits from the right and from the left are different.= does not exist (hey, I got one!!)
f(3) is on the graph, it's the red dot.(e) f(3)
The empty dot means that it's not a point of the function.