the charge placed at A=q

the chrarge placed at B=2q

the charge on C=Q

distance between them=d

let a particle C be placed at "x" distance from A.

if A and B should be at rest force avted upon A and B by C must be equal.

So force on A due to C=

So the force on B due to C=

equate these and get the value of "x".

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