The symbol you are looking at is sigma, which means sum of all. Because the number on the bottom is 1 and the top is 6, these are the start and end numbers for our n value. Therefore, it means to add together all of the values of your expression (n -3), for all of the values with n equaling between 1 and 6. So let's plug in all of the numbers for n they ask for and see what we have.
1
1 - 3 = -2
2
2 - 3 = -1
3
3 - 3 = 0
4
4 - 3 = 1
5
5 - 3 = 2
6
6 - 3 = 3
Now, when we add all of these numbers up, we look for the answer.
