\[b_{5} = 432;\ \ q = \sqrt{6}:\]
\[b_{5} = b_{1} \cdot q^{4}\]
\[b_{1} = \frac{b_{5}}{q^{4}} = \frac{432}{\left( \sqrt{6} \right)^{4}} = \frac{432}{36} = 12.\]
\[Ответ:12.\]