May I know the explanation of Q18 of the MCQ under Classwork? I have no problem in getting 3ba^2, but since the question asked for magnitude, may I know why should I include the negative sign?

Thank you, that is not properly worded. The magnitude is of course 3ba^2.  The intent was for students to use          E = -dV/dx so that the electric field points opposite the increase in potential as x increases. I will get rid of the word magnitude, then the answer will be (d) or -3ba^2.

Thank you so much!