[tex] a_{10} = a_{4} + 6r \\ 21 = - 9 + 6r \\ 6r = 30 \\ r = 5[/tex]
[tex] a_{4} - 3 \times 5 = a_{1} \\ a_{1} = - 9 - 15 \\ a_{1} = - 24 \\ a_{n} = 21[/tex]
[tex] S_{n} = \frac{ a_{1} + a_{n} }{2} \times n \\ S_{10} = \frac{ - 24+ 21}{2} \times 10 = \frac{ - 3}{1} \times 5 = - 3 \times 5 = - 15[/tex]
