Question 339231
look at each term:  from 9 to 7  we see a difference of -2
from 7 to 5 we see a difference of -2
Therefore the common difference d = -2

first term a = 9

nth term should be describe as a_n = a + d*(n-1)

formula :  a_n = 9 + -2*(n-1)

makes sense because the 2nd term 7 = 9 -2 *(2-1) = 7
try it out.