first two terms of an AP are a = - 3 and a_{2 =} 4.

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

a_{2 =} a + d

4 = - 3 + d

d = 7

Common difference, d = 7

a_{21 =} a + 20d

= - 3 + (20)(7)

= 137