Question

The first two terms of an arithmetic sequence are a1 = 2 and a2 = 4. What is a10,the tenth term?

Answer 1

a10=20

Explanation:

a1 = 2

a2 = 4

a3 = 6

........

a10 = 20

Multiply the subscript by 2 and you'll get your answer.

Answer 2

a(1) = 2
a(2) = 4

You can see that the terms are increasing by +2.
so

a(1) = 2
a(2) = 4
a(3) = 6
a(4) = 8
a(5) = 10
a(6) = 12
a(7) = 14
a(8) = 16
a(9) = 18
a(10) = 20

your answer is a(10) = 20.