Electrostatic Forces in Equilateral Triangle

[Mohamed Walid]

< Mentor Note -- New member is reminded to use the Homework Help Template when posting schoolwork questions at the PF >

Greetings everybody,
I've encountered a question about electrostatic forces on vertices of an equilateral triangle and I believe that I solved it correctly but my Physics teacher has marked it as incorrect. Am I correct? Amy clue why my Physics teacher marked it wrong? This is the question and the Physics teacher's marking of my answer. I attached it as well in case the link's dead.
https://s27.postimg.org/p7kkt5pbn/IMG_3511.jpg