Calculating Thevenin's circuit using KCL or node analysis
The classic method looks like this







simulate this circuit &ndash; Schematic created using CircuitLab

Step one 
Calculate Thevenin's equivalent impedance \$R_{TH}\$. Short all voltage sources and open circuit current sources. 

As you can see \$R_{TH} = R1+R2 = 12\Omega\$

Step two 
We need to find the \$V_{TH}\$ voltage for this circuit: 







simulate this circuit

$$V_{TH} = V1 + R1 \cdot I1 = 12V + 12V = 24V$$

Hence, the Thevenin equivalent circuit will look like this: 







simulate this circuit

So we can find \$ V_{AB} = 24V * \frac{4\Omega}{12\Omega+4\Omega} = 6V \$ and additional \$IR_3 = \frac{6V}{4\Omega} = 1.5A\$

As for the nodal equation 

The corrent equation looks like this 

$$\frac{V_X}{R_2+R_3} + \frac{V_X - V1}{R_1} - I_1 = 0$$