right, but how can i find Vx?Thevenin circuit is a resistor and voltage source in series.
To reduce the circuit to a thevenin equivalent circuit you would replace all the resistances between a and b with one equivalent resistor and reduce the voltage sources to one source.
think of two parallel resistors being represented as one resistor it's similar to this except you have a bunch of resistors and a dependent source as well.
as for the dependent source you shouldn't panic. if you can find the voltage vx then you know the voltage across the dependent source is just 5vx.