Which term of the A.P : 21, 18, 15, ............. is (-81) ?

a = 21

d = 18 - 21 = -3 .

Let -81 be the nth term.

Now, nth term, an= a+(n−1)d

-81 = 21 + ( n -1 ) -3

-81 = 21 -3n + 3

-81 = 24-3n

3n = 24 + 81

n = 105/3 = 35