Question

Two point charges of -86.5 mC and 36.3 mC are held fixed on an x axis at the origin and x = 24.4 cm, respectively. What are the magnitude and direction of the total electrostatic force on a third charge of 18.8 mC placed at
(a) x = 48.8 cm
(b) x = 97.6 cm

I've tried everything, and it seems I can't get the correct answers.

Answer 1

Do each individually, then add (or subtract) the two vectors
F = kq1q2/r^2
for example for a)
F = 9e9(86.5e-3)(18.8e-3)/.488^2 pulling towards negative x direction
now do it for the 36.3e-3 charge which also pushes towards the negative x direction.