The 9th term of an AP is 47 and its 16th term is 89. Find its first term.

$a_{16}-a_9=7d=89-47=42$, so $d=6$. Then $a+8d=47$ gives $a=47-48=-1$. Subtracting only 7$d$ gives 5, subtracting 9$d$ gives -7, and 6 is the common difference reported instead of the first term.