This is an arithmetic sequence because a common difference exists. The common difference is a constant found when taking any term and subtracting the previous term from it. In this case the common difference is 4, meaning that each term is 4 units greater than the previous term. Any arithmetic sequence can be expressed as:
a(n)=a+d(n-1), a=initial term, d=common difference, n=term number
In this case we know a=11 and d=4 so
a(n)=11+4(n-1) which can be simplified...
a(n)=11+4n-4
a(n)=4n+7
