Four charges q1 = q3 = -q and q2 = q4 = +q, where q = 9 µC, are fixed at the corners of a square with sides a = 1.3 m. (a) Calculate the x- and y-components of the net electric field at the midpoint M of the bottom side of the square. (b) Find the total force exerted on q4 by the charges q1, q2, and q3: (c) Find the force on a test charge Q = -0.4 µC placed at the midpoint M' of the top side of the square: I figured out that the Y-component of the net electric field at the midpoint M of the bottom side of the square is zero because the fields cancel out leaving only a field with an X component. To figure out the X component, I tried to use the law of superposition and add the electric fields like vectors. I used E=F/q and then E=(kQ)/r^2 on each of the charges to the left and right of the midpoint. Then, since the electric fields are moving in opposite directions, I subtracted the two fields to get zero, but apparently the X component of the field isn't zero. For part b, I drew the force vector diagram, so I have 3 vectors with one going left, the other going down and another going to the upper right. I don't know what to do next so any help would be appreciated here. For part c, it's a similar situation with part A and so the Y component is zero again. This time however, I used coulomb's law and the equation kQq/r^2. I plugged in numbers and got 0.0767. However I believed that the forces on the test charge were in the same direction and so I multiplied by 2 to get 0.1534 . This wasn't right either. Sorry for the long post and any help you can give me would be very awesome.