Find the 21st term in the arithmetic sequence 9 3 - 3

It decreases by 6 each time

first term is 9

an=a1+d (n-1)

a1=first term

d=common differnence

so

first term or a1 is 9

common differnce is - 6

an=9-6 (n-1)

21st term

a21=9-6 (21-1)

a21=9-6 (20)

a21=9-120

a21=-111

the 21st term is - 111