I am sorry for the late answer but i think i figured it out now. After you mentioned the equations, i researvhed a bit on google (because i didnt understand them in my book) and realized that i understood something completetly the wrong way and that's why i couldn't continue...so thank you
Its actually relly hard to put the question in english. But we need to use multiple things to be able to calculate that, for example the area of a Circle and the electric field constant. but i just don't know how everything ties together.