Finding expresion for current through resistor

1. Dec 17, 2008

leopard

I want to find an expression for $$I_5$$.

I see that $$I_{S1} = I_4 - I_5$$, but how can I proceed from here?

2. Dec 17, 2008

buffordboy23

What other info were you given for the problem? What are the resistances, etc.?

Just use Kirchoff's voltage and current laws.