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a: find the magnitude of the electric field and the direction at x= 0.

b: derive an expression for the electric field at points on the x-axis. use results to graph the x-component of the electric field as a function of x, for values x between -4a and +4a.

I would get the first part by using e=1/4pi(e0) * modulus q / r^2

so in this case i would get 2q/4pi(e0)a^2 .. right?

now how do i got about doing question b... don't even know where to start.