Find the Nth Term and the Indicated Term of the Arithmetic

Multiple Choice

Find the nth term and the indicated term of the arithmetic sequence whose initial term, a, and common difference, d, are
given.
- $$\begin{array} { l } \mathrm { a } = 9 ; \mathrm { d } = - 9 \\\mathrm { a } _ { \mathrm { n } } = ? ; \mathrm { a } _ { 13 } = ?\end{array}$$

A) $a _ { n } = 18 - 9 n ; a _ { 13 } = - 99$
B) $a _ { n } = 18 - 9 n ; a _ { 13 } = - 54$
C) $a _ { n } = 9 - 9 n ; a _ { 13 } = - 99$
D) $a _ { n } = 18 + 9 n ; a _ { 13 } = - 99$