Understanding equilibrium equations
Just a guess: it could be that you are dealing with some vector/matrix notation of nodal analysis or mesh analysis. 

Nodal analysis:


\$A\$ would represent conductivities (usually denoted by \$G\$), 
\$x\$ would be node potentials (usually denoted by \$v\$) and 
\$b\$ and \$e\$ would mean branch currents and (independent) current sources or vice versa (usually denoted by \$i\$). 


Mesh analysis:


\$A\$ would represent resistances (usually denoted by \$R\$), 
\$x\$ would be mesh currents (usually denoted by \$i\$) and 
\$b\$ and \$e\$ would mean branch voltages and (independent) voltage sources or vice versa (usually denoted by \$v\$). 


In both cases \$Ax = b - e\$ would in fact represent Ohm's law.