help on using maxima to solve systems of equations

Hello All,

I'm new to using Maxima. I tried to use the program to solve systems of 
equations, but I'm not succesful.
I've reduced my problem to a very simple example:

(%i1) e1:y=a*x+b;
(%o1) y=a*x+b
(%i2) e2:a=c;
(%o2) a=c
(%i3) solve(e1,a);
(%o3) [a=(y-b)/x]
(%i4) solve([e1,e2],a);
Inconsistent equations:  [1]
 -- an error.  To debug this try debugmode(true);
(%i5) solve([e1,e2],c);
Inconsistent equations:  [1]
 -- an error.  To debug this try debugmode(true);

Can anybody advise me what I should do to get a solution to such a 
system of equations?
(the real equations will be more complicated of course)

Thanks,
Tom