1. Find RTH by killing the sources.
RTH = 1 ohm + 2 ohm || 3 ohm.
RTH = 2.2 ohm.
2. Find Voc by circuit analysis.
Since we are looking for the open-circuit voltage, I would recommend nodal analysis. If we solve for V1 we will be most of the way to the complete solution since Voc = V1 + 2 V. The 2 V comes from the voltage drop across the 1 ohm resistor that has 2 A current through it from the current source.
3. Use Thévenin generator to complete circuit analysis.
The voltage across the 5 ohm resistor is (5.6 V) (5.0 ohm) / 7.2 ohm) = 3.9 V.
Thanks go to Eddie Knotts for reporting an error in way the current source
was killed in the original version of the figure in Part 1.