Let the nth term of the AP be the first negative term.

In the given AP –

first term(a) = 24 and,

common difference(d) = (n + 1)th term – nth term = 21 – 24 = – 3

According to question –

∴ a_n < 0

⇒ a + (n – 1) × d < 0

⇒ 24 + (n – 1) × (– 3) < 0

⇒ – 3n + 27 < 0

⇒ 3n > 27

∴ n > 9

Thus, the first negative term of given AP is 10th term.