The given sequence is 1, 6, 11, 16, 21.
As you can observe, the difference in the sequence is 5 (d = 5).
The first term is 1. So, we have to use an arithmetic sequence formula
[tex]a_n=a_1+(n-1)d[/tex]In this case, the equation would be
[tex]a_n=1+(n-1)\cdot5[/tex]The simplified equation would be
[tex]a_n=5n-4[/tex]