(a)
[tex]r = \frac{u_{4}}{u_{3}} = \frac{ - 56}{28} = - 2[/tex]
(b)
[tex]u_{n} = u_{1} \times r {}^{n - 1} \\ ie \colon \: u_{4} = u_{1} \times r {}^{4 - 1} \\ \: \: \: - 56 = u_{1} \times ( - 2) {}^{3} \\ u_{1} = \frac{ - 56}{ - 8} = 7[/tex]
(c)
[tex]S_{n} = ( \frac{1 - r {}^{n} }{1 - r} ) \: (u_{1}) \\ S_{6}=( \frac{1 - ( - 2) {}^{6} }{1 - ( - 2)} ) \: (7) \\ S_{6}= - 147[/tex]
