What's wrong with my answer finding the resistance?
I think I found it:

The mistake happened in Step 3: it should be:

$$
-(-2) + V_{src} + 2\cdot I = 0 \\
2 -6 + 2\cdot I = 0\\
I = 2\mathrm{A}
$$

Then, step 4:

$$
1 - 2 - I_1 = 0 \\
I_1 = -1\,\mathrm{A}
$$

Then, step 5:

$$
R = \dfrac{U}{I} = \dfrac{-2}{-1} = 2 \,\Omega
$$