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.
Have you learnt about arithmetic progressions? If not check out JG89's suggestion. If you have learnt about APs the you should know the sum of a AP formula.
ok i got that question now i dont know how to do this one
100
E (5^(i) - 5^(i-1))
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?