\[\boxed{\mathbf{839\ (839).\ }Еуроки\ - \ ДЗ\ без\ мороки}\]
\[b_{1} = 6;\ b_{4} = 750\]
\[b_{4} = b_{1} \cdot q^{3}\ \]
\[q^{3} = \frac{b_{4}}{b_{1}} = \frac{750}{6} = 125 \Longrightarrow \ \ q = 5\]
\[b_{2} = 6 \cdot 5 = 30\ \ \ \ \ \]
\[b_{3} = 30 \cdot 5 = 150\]
\[Ответ:30;\ 150.\]