Question

how do i use sigma notation to write the sum?

Answer 1

Assuming that E is the sigma notation, you do:
end number
E (formula. Like n + 5 = x)
begin number

Answer 2

Imagine that end number/ begin number are like exponents and subscripts.

Answer 3

So....
3
E (n+5) would be (1+5) + (2+5) + (3+5) = 21 if I added correctly.
1