The simulation on the left represents a positive charge (+q) near two infinite grounded conducting sheets. The animation on the right depicts a similar situation without the conductor present. The rectangles are there for your reference only.
Use the animation on the right to place charge(s) where they belong to make the field on the right match the field on the left for the middle region, 0.75>x>-0.75 (the region where the +q is located). Assuming you could do this, where are the charge(s)?