How I find the current and voltage in this matrix?
In this case, you first want to reduce the circuit.  I'm going to have to explain this without pictures, sorry.  But I think it'll be pretty clear anyway!


The first thing to do is to ignore the small arrow marks that are showing the branch currents - they are no longer accurate once the circuit changed.  Neither are V1 and V2.
Look at the 6k resistor.  It has effectively been removed from the circuit.  That is, there will be no current through this resistor.  There are a few ways to see why this is so. Here are two of them:


The resistor is in parallel with a zero-Ohm path.  Any current flow will happen through this short, and nothing will flow through the more difficult path of the resistor.
There can be no voltage that exists across the short.  Therefore, no voltage will be presented across the 6k resistor, either, since they are in parallel.

So, now, you are left with the 1mA source, flowing though a 6k resistor in parallel with 12k resistor.  Easy to analyse with either of the techniques you mentioned.


Good luck :)