Write a Formula for the General Term (The Nth Term) $$\mathbf { a } _ { \mathbf { n } }$$

Multiple Choice

Write a formula for the general term (the nth term) of the arithmetic sequence. Then use the formula for $$\mathbf { a } _ { \mathbf { n } }$$ to find $$a _ { 20 }$$ , the 20th term of the sequence.
- $21,12,3 , - 6 , \ldots$

A) $a _ { n } = 30 - 9 n ; a _ { 20 } = - 150$
B) $a _ { n } = 21 - 9 n ; a _ { 20 } = - 159$
C) $a _ { n } = 9 n - 30 ; a _ { 20 } = 150$
D) $a _ { n } = 9 n - 21 ; a _ { 20 } = 159$