Originally Posted by cse63146
Q = (E*r^2)/K
makes sense to me, but should it be negative or positive?
Remember that the vector direction of electric field is defined as radially outward
from a positive charge and radially inward from a negative charge. The point P is north of the charge and the field vector there points back southward toward the charge. So what sign would the charge need to have? (BTW, I agree with your charge magnitude.)