Apply the definition of "arithmetic sequence". What is the difference between the 13th and 7th terms? What then must be the value of the common difference d?
Once you have the value of the common difference d, plug this into the formula for the n-th term, a_n, with n = 7, and solve for the value of a_1.
If you get stuck, please reply showing how far you have gotten. Thank you.
Eliz.