Click on Equation at the top. Unfortunately this formula would simply add all the numbers to the left of the cell ie. So whats the best way to insert these relatively difficult formulas.
You can also select Insert New Equation of course.
Go to Insert tab. After you insert the equation the Equation Tools Design tab opens with symbols and structures that can be added to your equation. Here are 3 commonly used methods you can try. In the Formula box check the text between the parentheses to make sure Word includes the cells you want to sum and click OK.