I have a cube with sides l and therfore a total volume of l^3. The E-field is ax + by + cz where a, b, and c are known constants. It says to find the total E-field. The answer is (a + b + c)*l^3. I have no idea how they get this. I tried summing up all of the sides separately bit I kept getting zero. I understand why it is zero and do not understand the answer in the book. How do you do it? Thanks for the help!