I need to find equivalent circuit :

Ive solved it for Req, which is 3Ω, but how to find the equivalent Voltage?
Please help me, it will rescue my studies

 

I'm assuming you need to find Vth across a and b, right? I.e. the 1ohm is the load resistor...

Basically all you have to do is calculate the voltage at a with respect to point b when the load resistor is no longer connected.

So just do nodal analysis again, but leave the 1ohm out and an open circuit between a and b.

http://en.wikipedia.org/wiki/Thevenin's_theorem

 

Proceed in a manner similar to what I showed you in your other post, solving the network with the nodal method. Delete the 1 ohm resistor while solving.

Let the bottom edge be your reference node, and number the nodes on the top edge left to right: v1, v2 and v3.

Since this is homework, I'm not going to do the whole thing for you again, but I will get you started and then you show your further work.

The first equation will be a constraint equation, saying that v1 = 12 volts.

1*v1 +0*v2 + 0*v3 = 12

Your second equation should express the fact that the sum of the currents into node v2 is zero, and the same for node v3.

Show an attempt to get all three equations, and if you run into trouble, more help will be forthcoming.