What is the Electric Field in a Ball with a Positive Charge at R/2?

[dannee]

Homework Statement

Positive charge Q uniformly distributed in a ball with radius R, which is located at the beginning of X axis. Positive point charge Q is located on the X axis on R=2. what is the size of the electric field in R/2?

Homework Equations

The Attempt at a Solution

i've tried to use the equations of electric field, K*(Q/r^2), once put the distance of charge Q with R=3/2, adding it to the electric field of the ball on the point, which is K*(Q/a^3), as a=0.5R. but the correct answer is K*(1/18)*(Q/r^2). where do i go wrong?

 

[Mindscrape]

Are you sure that the problem says on the axis at R=2, not X=2R? The wording of the problem you put up is really bad, maybe you could post exactly what the problem you were given says.