Here, a = 121 and d = 117 – 121 = – 4.
Let, a_n be the first negative term of this AP.
⇒ a_n < 0
⇒ a + (n – 1)d < 0
⇒ 121 + (n -1)(-4) < 0
⇒ 121 – 4n + 4 < 0
⇒ 125 125/4
⇒ n > 31.25
⇒ n = 32
Hence, 32nd term of the AP: 121, 117, 113 … is its first negative term.