Here, First term = a = 20

Common difference = d = 17 - 20 = - 3

To find = 35^th term, ∴ n = 35

Using the formula for finding n^th term of an A.P.,

a_n = a + (n - 1) × d

∴ a_n = 20 + (35 - 1) × (-3)

⇒ a_n = 20 + 34 × (-3) = 20 - 102 = - 82

∴ 20^th term of the given AP is - 82.