superposition analysis technique
Noting your edit, which changes the \$V_2\$ voltage value, look at the following:







simulate this circuit &ndash; Schematic created using CircuitLab

You can go directly from \$A\$ to \$D\$ by simply using a voltage divider approach. But, following your conversion first to Norton, then applying the two resistors in parallel, and then converting back to Thevenin (\$A\to B\to C\to D\$) gets you to the same place. (Notice how these magnitudes differ from yours.)

At this point, I might do this:







simulate this circuit

From here it is easy to compute the current in \$R_1=\frac{6\:\text{V}-19.2\:\text{V}}{4\:\Omega+12\:\Omega}=-825\:\text{mA}\$, where the green arrow shown is the conventional current direction.