solution: part c.

First we start with the drawing:

From the drawing we can see that the fields that the two charges create at this point are not equal and will not cancel in any way. We need to find each of them seperately and then add them up.
Start with the field by charge a:

This is pointed straight up as seen in the figure.
Now to move on to the field created by charge b. First we need to find the distance from the charge that point 3 is located and then plug that into the equation to find the magnitude of the field:

To find the angle above the horizontal that this field points, we find the angle that the point is at in comparison to charge b:

Now I break up into its components so that I can add up and :

Now for adding up the two vectors, we take the components of each one and add them seperately.

The x-components are easy, as doesn't have an x-component, it's just the x-component of

Now for the y-components:

Now that we have the components, the final magnitude and angle can be found:

The electric field at point 3 is 7.8N/C pointed 81 degrees above the y direction.