Answer:
At centroid
Explanation:
In the given equilateral triangle ABC with side of 1 m. The three equal charges e,e,e are placed at the A,B and C.
And the fourth charge 2e is put at point O which is called centroid.
Now we can calculate the distance AD by applying pythagorean theorem as,

Put the values and get.

Now calculate AO as,

And the sides BO=CO=AO.
Now Force can be calculated as

And similarly,

Now we can calculate resultant of
in upward direction. as,

Therefore the resultant force on centroid O.

Therefore the fourth charge 2e should be placed on centroid so that it experience zero force.