Question 830793
Your sequence is:

(x, 2x + 1, 21)
You can work this backward.
Find the middle number first.
21 - 1 = 2x
20 = 2x
10 = x
.
Now take the 10 and work backward with the same formula.
10 - 1 = 2x
9 = 2x
4.5 = x
.
Let's plug it in to make certain.
4.5 * 2 = 9, plus 1 = 10
10 * 2 = 20, plus 1 = 21
Success!