This is an arithmetic sequence because each term is 9 more than the previous term, called the common difference. The equation for an arithmetic sequence is:
a(n)=a+d(n-1), a=initial term, d=common difference, n=term number...
In this case a=9 and d=9 so
a(n)=9+9(n-1)
a(n)=9+9n-9
a(n)=9n
So for any term n, a(n)=9n
...
a(16)=9*16=144
