how to find Thevenin Equivalent Voltage and Thevenin Equivalent Resistance?
I think that simple circuits like this should be simply solved by inspection.
Let's note the node A in figure.

Now, looking the right side of the circuit, from A down to (-) terminal, you can note a parallel of 6//(4+2) = 3 ohms.
At this point, considering the left side too, we will have a very simple circuit with 9 V and two 3 ohms series resistance; therefore, the voltage on point A is 9/2 V.
Knowing the voltage at node A, it's easy to calculate the voltage level Vo, considering the voltage divider : Vo = (9/2)* 4/(2+4) = 3 V