DUDE, take any distance x and find the force on the charge in terms of x.
Then diffrentiate it in terms of x and put it equal to zero and find x .
Then after finding x , put it in the force eqn to find the maximum force..!!
CHEERS ..!!
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Courage to invent,
Courage to travel into an unexplored path,
Courage to discover the impossible,
Courage to combat the problems..