Answer:
The charge must be negative
Explanation:
Hi!
The direction of the electric field of a charge depends on the sign of the charge, when it is positive, the electric field points outwards the charge but when it is negative, it points towards the charge. (As you can seen on teh attached image)
So, if on the faces of the box, the non-zero elecric field points inwards, it must mean that the total charge inside the box must be negative
