global variables (ones that belong to the global scope)
  a  |  b  |  c  
------------------
  3  |  4  |  5      # initial values of these variables
  3  |  9  |  5      # b = mystery(c, a)
                         = mystery(5, 3)
                         = 9

local variables (ones that belong to `mystery`)
  a  |  b  |  c  
------------------
  5  |  3  |         # the call is mystery(5, 3), so a = 5, b = 3
  5  |  3  |  9      # a > b, so c = a + 4
                                   = 5 + 4 = 9

output
------
5 3       # printed by mystery
3 9 5     # printed by the final statement in the program


c is still 5 at the end of the program because there are two different
variables called c -- one that belongs to mystery, and one that belongs
to the global scope. The global c is assigned 5, and after that it is never 
changed. 

The local c (the one that belongs to mystery) is assigned 9, 
but that doesn't change the global c.