Finding the maximum current
This question is probably related to the maximum power transfer theorem that states that: " to obtain maximum external power from a source with a finite internal resistance, the resistance of the load must equal the resistance of the source as viewed from its output terminals".

In reality, all sources of electrical power have an output resistance (or impedance in a more general sense) to compensate the limited power available. So, in order to maximize power, the load resistance RL "must be" equal to the source resistance (not shown in your circuit).

If you want an deeper explanation on this I would recommend the book by Hayt.

I hope this helps.