Answer:
For an nth term in an arithmetic sequence
U(n) = a + ( n-1)d
a = first term
d = common difference
n = number of terms.
Considering the above picture
a = 9
d = 6 - 9 = - 3
For an nth term of the sequence
U(n) = 9 + (n - 1)-3
= 9 - 3n + 3
= 12 - 3n
Hope this helps
