Generate the first few terms to see what's going on.
Put in i=1, i=2, i=3,...
The terms are -3, -8, -13, -18 etc. and you want to add these up.
The numbers form an AP (arithmetic prgression) so find a and d, and use the sum of an AP formula.
ok i got that question now i dont know how to do this one
E (5^(i) - 5^(i-1))
Do you know what a telescoping sum is?
It means that all of the middle terms cancel leaving only a couple at the beggining and at the end. Think about your problem, which terms are gonna cancel other terms, and what are the only terms that will remain standing?