Let nth term of the given AP be 210.

Here, first term, a = 21

and common difference, d = 42 – 21 = 21 and a_n = 210

As we know, nth term of an AP is

a_{n =} a + (n - 1)d

where a = first term

a_n is nth term

d is the common difference

210 = 21 + (n - 1)21

189 = (n - 1)21

n - 1 = 9

n = 10

So the 10th term of an AP is 210.