How can I find Thevenin Equivalent Voltage in the given circuit?
Hint: The easiest method for this case is to use superposition. Turn off all independent sources except one and calculate the voltage across a and b. Repeat until you've found the contribution for all sources. Add the contributions from each individual source to get the Thevenin equivalent voltage for all of them.

Remember that turning off a voltage source means replacing it with a short circuit, and turning off a current source means replacing it with an open circuit.