Define node voltages and apply Kirchhoff's Current Law (KCL).

Here's the solution for the circuit problem:

Step 1: Define node voltages and apply Kirchhoff's Current Law (KCL).
Let the potential at node N be $V_N = 0 \, \text{V}$ (ground). Let the potential at node M be $V_M$.
We will define currents flowing out of node M as positive.
The central branch contains a 4 $\Omega$ resistor, a 1 V battery (with 2 $\Omega$ internal resistance). The total resistance in this branch is $4 \, \Omega + 2 \, \Omega = 6 \, \Omega$. The 1 V battery's positive terminal is towards N, so it opposes current flowing from M to N.
The current $I_A$