What is the the sum of i=1 to 7 for x1=3 (how do i do this). Sum of is that funny E thing that i don't get

Answer 1

the first thing, is that it is not a funny E,
it is a Sigma, which is a greek letter simmliar to S.
the second thing, i do not understand what is what you are trying to add:

sum i=1 to 7 of what?, of 3?
3+3+3+3+3+3+3 =7(3)=21

Answer 2

What function (or expression) are you summing?
e.g.
sum(i), i=1 to 7 = 1+2+3+4+5+6+7

what is the relation of x1=3 to this?