Answer
$a_n=2n+1$
Work Step by Step
Our aim is to determine a formula for the general term of the arithmetic sequence $a_n$.
Such as: $a_n=a_1+(n-1)d$
we have $a_3-a_8=a_1+7d-a_1-2d$
or, $17-7=a_1+7d-a_1-2d$
or, $d=2$
Thus, $a_1=7-d=7-4=3$
Now, $a_n=a_1+(n-1)d \implies 3+2n-2$
or, $a_n=2n+1$
