You need an expression for the power delivered to the load (not counting power lost in the 4 ohm resistance of the battery). I suggest you begin by calling the resistance of the network Z. Then the current through Z is I = V/(Z+4). The power delivered to it is P = I²/Z.
That function of Z will vary with Z and have a maximum for some specific value of Z. You could use the derivative method to find that value.