How to solve this exercise to find maximum average power
To find the Thévenin voltage, Vth, take ZL out and then find the voltage that is developed between A &amp; B due to your source (where A &amp; B are the terminals of ZL in your drawing).  Vth is also known as the &quot;open circuit&quot; voltage.
To find the Thévenin impedance, Zth, kill the source, remove ZL, and then find the equivalent impedance between points A &amp; B.
Read about Thévenin here.