$L=2\pi R\times N\Rightarrow N=\frac{L}{2\pi R}$ $B=\frac{{{\mu }_{\circ }}}{2}\frac{NI}{R}\Rightarrow \frac{{{B}_{2}}}{{{B}_{1}}}=\frac{{{N}_{2}}}{{{N}_{1}}}\times \frac{{{R}_{1}}}{{{R}_{2}}}=\frac{\frac{L}{2\pi {{R}_{2}}}}{\frac{L}{2\pi {{R}_{1}}}}\times \frac{{{R}_{1}}}{{{R}_{2}}}={{(\frac{{{R}_{1}}}{{{R}_{2}}})}^{2}}={{(\frac{6}{9})}^{2}}=\frac{4}{9}\Rightarrow {{B}_{2}}=18\times \frac{4}{9}\Rightarrow {{B}_{2}}=8G$