13) Find the order of each of the following elements in the multiplicative group of units U(sub)p.
a.  for p=13
b.  for p=13
c.  for p=17
d.  for p=17
If you can tell me what units mean, and how to solve at least one of the problems, I think I should be able to do the rest. At this point, I don't know where to begin.