Heating water

A resistor of resistance $$R$$ is connected to a power supply of constant EMF $$\mathcal{E}=12~\mbox{V}$$ and internal resistance $$r=3~\Omega$$ . The energy dissipated in the resistor $$R$$ is used to heat water. What resistance $$R$$ in Ohms gives the maximum power dissipated in the resistor?

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