Arithmetic sequence is modeled by the next formula:

where an is the nth term, a1 is the first term, and d is the common difference.
In the case of the first sequence:

In the case of the second sequence:

The 6th term of the first sequence is:

The 5th term of the second sequence is:

The 11th term of the first sequence is:

The 9th term of the second sequence is:

Therefore, the first three common terms are: 1, 21, and 41. And the sum of them is 63