Algebra 2 Please help...?

Question

Ok.. so i have the answer key b/c it is a review for exams, but i can't figure out how to solve it. Please help..
Given the series 3+7+11+....+307.
a. Identify the series as arithmetic or geometric.
b. rewrite the series using E (funky sum thing)- notation
c. Evaluate the series.

Answer 1

a. arithmetic - goes up by 4 each term

(307+1)/4 gives which term of the series you're on, so 77

E (from 1 to 77) 4n-1

evaluate = number of terms * average of first and last terms
E (from 1 to 77) 4n-1 = 77 * (3+307)/2 = 77 * 310/2 =
77 * 105 = 8085