Series is 15, 12, 9…,
Difference between terms is 3
By Arithmetic progression
a_{n}=a_{1}+(n-1)d
a_n = the nᵗʰ term in the sequence
a_1 = the first term in the sequence
d = the common difference between terms
So nth term is a(n) = 15 + (n-1)*3
= 15+ 3n -3
= 12 + 3n