When you have an arithmetic sequence you can use the next general formula to get the explicit formula:
[tex]a(n)=a(1)+d(n-1)[/tex]Where a(1) is the first term in the sequence, d is the difference between each term in the sequence, and n is the nth term
You have the sequence: 28,38,48,58
The difference in this sequence is d=10
The first term is: a(1)=28
Then:
[tex]a(n)=28+10(n-1)[/tex][tex]a(n)=28+10n-10[/tex][tex]a(n)=18+10n[/tex]Then, the explicit rule for the given arithmetic sequence is: a(n)=18+10n