I seem to be having some trouble grasping this concept. I want to know how I can tell if a limit exists by looking at a graph (cartesian plane) given specific points.

For instance I have the problem:

Given the points (0, -3) [its an open dot at this point] and (-2, -5) [a closed dot here] and y = h(x) determine the following limits (if they exist):

a. lim h(x)

x -> -2

b. lim h(x)

x -> 0

What math techniques can I use to approach a problem like this?

Thanks for any help.