Negative charge inside the cube towards which 5 N/C electric field is entering.
Work Step by Step
The way that we're going to solve this is by looking at the electric field that's coming into the cube, and going out of the cube. We will add up all of the quantities that are going in, and compare that with the sum of the quantities that are exiting the cube. $$E(out) = 45 N/C$$ $$E(in) = 50 N/C$$ $$E(net) = 5 N/C$$ This implies that there should be a negative charge inside the cube towards which 5 N/C electric field is entering.