Please type your work into the text entry box. Having to open an attachment is a pain in the butt, since I have to read your work on one window and post comments in another. It's much better if everything is in one place.
Tn = a + (n - 1)d
In the next line you have T25 = a + (15 - 1)d
Why do you have 15 on the right side?
In the 3rd line you have
25 = a + 14d
Where did the 25 come from?
If you want help, please post the complete problem statement, using the standard template, together with your work.
GRRR! Now I understand a little better what you're doing. You have part of the information in your first post and part in the attachment. If you had put all the information and your work in your post it would have been harder to miss.
The problem statement, as I see it is this:
Tn = a + (n - 1)d. The second term in this sequence is 6, and the 15th term is 25. What is the first term in this sequence?
From the given information,
T(2) = a + (2 - 1)d = 6, and
T(15) = a + (15 - 1) d = 25
So the problem boils down to finding the solution of
a + d = 6
a + 14d = 25