Question

how to get the equation to find vth in Thevenin’s equivalent circuit

Answer 1

Depending if you have circuit or not, if you have circuit
Open all current sources
Short all voltage sources
Combine components looking into your two ports
This is the Thevinin impedance
The Thevinin voltage is the open circuit voltage at the two ports
If a black box
The Thevinin voltage is still the open circuit voltage at the two ports
You then add a resistor across the ports and measure port voltage, with this you can calculate the Thevinin resistance.
Ensure resistance across port is large enough not to short out black box.