Which Term of AP is Zero?

Given Arithmetic Progression (A.P.): 21, 18, 15, ... First term, $a = 21$. Common difference, $d = 18 - 21 = -3$. We want to find which term is 0. Let the $n^{th}$ term be 0. The formula for the $n^{th}$ term of an A.P. is $a_n = a + (n-1)d$. Set $a_n = 0$: $0 = 21 + (n-1)(-3)$ $0 = 21 - 3n + 3$ $0 = 24 - 3n$ $3n = 24$ $n = 8$ So, the 8th term of the A.P. is zero.