Actually there are an infinite number of combinations of possible values for a,b, and c. There's no way to get a definate answer unless another equation is given (like for example, a + b = c or a + b = 3 or something)
Heck, I could do:
a = 2
b = 2
c = 1
or
a = 4
b = 4
c = 2
a = 8
b = 8
c = 4
and go on like that until I'm 1,000 years old.
I think the problem just asks to solve for b in terms of a and c. There's really nothing else one could do with such little information.
-Warik