$V = 4 - Ir \Rightarrow 18 - 2(I) = 12\,\,\,\,\,\,\,\,\,\,\,I = 3A$$I = \frac{4}{{R + r}} \to 3 = \frac{{18}}{{{R_T} + 2}} \to {R_T} = 4\Omega $$ \to R = 14\Omega $