fni (looks like a big E)x for the following group 
of terms
-2,3,-3,6,-5,8,-4 

A)6
B)2
C)3
D)-1

I assume you meant this" "Sx".
That S is the Greek letter "sigma". 
It is found in the the name of many college and 
university fraternities and sororities. It is the
Greek letter "S" to remind us of the word "SUM" or
"SUMMATION".  When it is written before a variable
it means to find the SUM of all the numbers that x
could stand for.

Sx = (-2)+(3)+(-3)+(6)+(-5)+(8)+(-4) = 3

Edwin