X Kq F Help Me ASAP: Confusion Between 2 Equations For Force?

Help me ASAP: Confusion between 2 Equations for Force? - x kq f

I forgot to Q1 as between F = qE and F = K (Q2) to distinguish (r ^ 2). I do not know which one to use when. Here is an example of a problem can not be understood: "The positive charge Q is uniformly distributed along the x-axis at x = 0 and x = a. A positive point charge 'q' is on the x-axis at x = a + R , a distance "on the right side at the end of Q (a) Calculate the x and y components of the electric field generated by the dot distribution of charge Q on the x-axis postiive where x> a." The answer to this question is e_x = KQ / a [(1/xa) - (1 / x)], and I understand. But my problem is at point B, where it says: "Do you find the strength (size and address), the charge distribution Q exerts on q. "Here they use F = QE, but I do not understand why we can not use F = K Q1 (Q2) / (r ^ 2). I really appreciate it if you could please explain, all this because my exams is tomorrow. Thank you.