Question 918942
{{{a[n]=-8+(n-1)d}}}, index being n, common difference being d.


Ratio of seventh term to ninth term is 5:8,
{{{(-8+(7-1)d)/(-8+(9-1)d)=5/8}}}
That is the key equation to form; and solving it should be a comfortable process.  You basically want to determine the value of d, common difference between terms.