Answer
$a_n=9+(n-1)6$
Work Step by Step
The nth term of an arithmetic sequence can be obtained by the following formula: $a_n=a_1+(n-1)d$, where $a_1$ is the first term and $d$ is the common difference.
Hence here: $a_11-a_4=7d\\42=7d\\d=6$
Then $a_4=(4-1)d+a_1\\27=3\cdot6+a_1\\a_1=9$
Thus $d=6,a_1=9,a_n=9+(n-1)6$
