It's an arithmetic sequence:
[tex]a_1=4;\ a_2=10;\ a_3=16;\ a_4=22;\ ...[/tex]
The explicit rule:
[tex]a_n=a_1+(n-1)d[/tex]
[tex]d=a_{n+1}-a_n\to d=a_2-a_1=a_3-a_2=a_4-a_3=...[/tex]
[tex]d=10-4=16-10=22-16=6[/tex]
Substitute:
[tex]f(n)=4+(n-1)(6)=4+6n-6=6n-2[/tex]
Answer: f(n) = 6n - 2
