Don't try solving this unless you're a physics wiz!?

Question

A disk of radius a = 18.8 cm lies in the yz plane with its axis along the x axis and carries a uniform surface charge density sigma = 2.7 nanoC/m2. Find the value of x for which Ex = 76.3 N/C.

Answer 1

Write down the eqation describing the x-component of the coloumb force from a point on the x-y plance, to a point on the x-axis (I'm assuming that is what the question is asking about.) Use Coloumb's Law as a starting point. You will want to do this in a cylindrical coordinate system to make this problem easiest. Then integrate that equation you just wrote down over the disk.  

Answer 2

Ok, unfortunately, E&M is difficult to type out here so i cannot show all of my work.

First you need to find the electric field along the x-axis of a uniform disc.  I've worked this out many times.  You should get:

E = ( (sigma)*x ) / (2 e0 ) [ 1/x - 1/sqrt[ a^2 + x^2 ] ].

Where e0 is epsilon.  You know sigma, you know "a".  Set this equal to 2.7 nanoC / m^2 and solve for x.  **********But be sure to convert all units.  Make sure you convert nanoC to C, and cm to m.

This should be a start.  Normally I would show you all of the work, but it benefits you more if you work it out yourself.  And besides, it builds character =).

Good Luck, hope this helped.