Answer
Missing electric field vector points into the face and $E = 5 N/C$.
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.
Thus, the missing electric field vector points into the face and $E = 5 N/C$.
