\[b_{1} = - 2;\ \ \ \ \ q = - 3:\]
\[b_{2} = - 2 \cdot ( - 3) = 6;\ \ \ \ \ \]
\[b_{3} = 6 \cdot ( - 3) = - 18;\ \ \]
\[b_{4} = - 18 \cdot ( - 3) = 54.\]