Which term of the AP 21, 18, 15,... is zero?

In the given AP, first term, a = 21 and common difference, d = (18 - 21) = -3
Let's its nth term be 0.
Then Tn = 0
 ⇒ a + (n - 1)d = 0

$\Rightarrow 21+(n-1) \times(-3)=0$

$\Rightarrow 24-3 n=0$

$\Rightarrow 3 n=24$

$\Rightarrow n=8$

Hence, the 8th term of the given AP is 0.