Ответ:

[tex]S_{n} = \frac{b_{1}( {q}^{n} - 1 ) }{q - 1} [/tex]

[tex]S_{3} = \frac{6( {( - 10)}^{3} - 1) }{ - 10 - 1} = \frac{6 \times (- 1001)}{ - 11} = 6 \times 91 = 546[/tex]