Answer:
a20 = -99
Step-by-step explanation:
hello :
the nth term in this arithmetic sequence is : an = a1 + (n -1 )d
a1 = 15 a2 = 9 d = a2-a1 = 9-15 = -6 ( common difrence)
an = 15 + (n -1 )(-6)
an = 15+6-6n
an = -6n+21
the 20th term when n = 20 : a20 = -6(20)+21 = -99
