First differences are ...
41 -39 = 2
43 -41 = 2
45 -43 = 2
These are all the same, so 2 is the common difference of the arithmetic sequence beginning with 39.
The Nth term is
[tex]a_{n}=a_{1}+(n-1)d\\a_{n}=39+2(n-1)\\a_{n}=37+2n[/tex]
The problem statement seems to ask for the 10th term and for a15. Here they are.
[tex]a_{10}=37+2\cdot 10=57\\a_{15}=37+2\cdot 15=67[/tex]
