What charge needs to be placed in order to make the potential difference zero?

Two charges are placed at the far corners of a rhombus 45 degree. One charge, +4uC is fixed to one corner and another, -6qC is fixed to the opposite corner. What charge would need to be placed at the intersection of the diagonals of the square in order to make the potential difference zero at each of the two unoccupied corners? Exam is tomorrow!