KVL with current source question
You should use KCL at the top and bottom nodes. Assuming there is nothing connected at a and b this should be easy.

You can't write a KVL equation with a voltage term for a current source, because the voltage across a current source is determined by the rest of the circuit. And you can never, ever talk about the "equivalent series resistance" of an ideal current source. However, in a case like this it allows you to determine one of the currents in the circuit by inspection.