Circuit analysis

Problem and my attempt at the solution are in the figure below (red is my work):

http://dl.getdropbox.com/u/364603/elec.png [Broken]

 

Yes, I get 300 V and 50 ohm on the right then (if I remember correctly to do that) and this gives me 3A running. But, can you spot any mistake in my equation.

It works good if I use -6, that gives V = 150 V or if I use other methods but I remember making this same mistake a year ago. I think I am missing something fundamental. I don't want to repeat it when solving complex circuits.