HELLO,
Free body diagram of charge +Q is as shown below.
Charge at A attracts charge +Q with force \(F_A\) and charge at B repels the charge +Q with force \(F_B\).
\(F_A= \frac{kqQ}{(r/2)^2}\) and \(F_B= \frac{kqQ}{(r/2)^2}\)
As both the force is in same direction and equal in magnitude so resultant of these force is double of one of force.
\(F_{res}= \frac{2kqQ}{(r/2)^2}\) in the direction of charge -q.
\(F_{res}= \frac{8kqQ}{r^2}\)
I hope you will understand.