Diode from the circuit can be removed as the battery with higher potential is make the diode reverse biased.
So forR=12Ω
−4V−6V+12Ω×I+12Ω×I=0
I=1024A
So forR=48Ω
−4V−6V+48Ω×I+12Ω×I=0
I=1060A