Point charges of +2.00 nC and - 5.00 nC are arranged on two of the vertices of an equilateral triangle. Find the magnitude and sign of the third charge which must be placed at the 3rd vertex of the triangle so that the potential at a point on the side midway between the +2.00 nC and - 5.00 nC is equal to zero. You do not need to know the size of the triangle.