Start with a diagram. Show the rod, the coordinate axes, and the point
*P*. Make your diagram BIG.

Step I: Model the system (the rod) as a collection of differential elements.
Draw a *typical* element on your diagram. Each element should correspond
to a differential change in one coordinate.

See diagram above

Step II. Identify a typical element and describe it using your coordinates.

The element extends from *x* to* x*+d*x* and has charge
d*q* = l d*x*

Step III. Express the contribution of your element to the desired sum.
(i.e. find the electric field d**E** produced by this element. Give
both its magnitude and its direction. With vectors, it is often easier
to calculate each component separately.

As we can see from the diagram, the electric field is in the *x*-direction,
and from Coulomb's law

Step IV. Find the limits of the integral.

The limits are *x* = -*a* to* x* = *a*

Step V: Integrate!