How do I calculate load resistance?
From the look of the question, it looks like all you were asked to do is to solve the circuit into the Thevenin equivalent. By that, it means reducing the circuit so it just has Rth and Vth with a load.

An example of a Thevenin equivalent circuit:



The image is taken from HERE. The page also has an example of solving a circuit to a Thevenin equivalent.

You don't need to do any more. The criteria of the question as written has been satisfied.