Question 1039329
This is an arithmetic sequence with common difference d, which can be calculated as follows:
Number of terms from a2 to a9: 7, so the total difference for 7 terms is 
{{{7*d=39-15=24}}}
Similarly, from a9 to a16 we have again 7 terms, so the total difference will again be 24.
Therefore: {{{a16=a9+24=63}}}