How do do this short circuit question / ( do i use thevinin and how ?)

The question is find the voltage( Voc) ;

http://www.flickr.com/photos/84781786@N0…
Do I use mesh ?

As there is no load at the output we do not need to create an equivalent circuit ( but we may choose to do so. )
The total EMF around the loop is 22 V and the total resistance is 4.5 kohm
Giving a current of 22/4500 = 0.0049 amp

The emf at point can be calculated using either arm
either -12 + 3300 *22/4500
0r 10 - 1200 *22/4500

Which both give +4.133 V as the open circuit EMF.

To create the full equivalent circuit you must determine the current flow if you short AB and can then give the effective series impedance of the power supply.
10/1200 - 12/3300 = 0.004697 A ( this is the short circuit current. )
and at that point as E/ I = R
then Req = 4.13333/0.004697 = 880 ohms as the equivalent series resistance in a Thevenin equivalent circuit.

1. There is no short circuit.
2. Use mesh