Given: 7^th term is 34 and 8^th term is 64

To find: find its 18^th term

34 = a + 6d ………….equation1

64 = a + 12d ………… equation2

Subtract equation1 from equation2 we get

d = 5

Put in equation1 we get

a = 4

So 18^th term is

4 + 17 × 5 = 89