Answer
5 N/C electric field into the front face.
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.
$$E(in) = 35 N/C$$ $$E(out) = 40 N/C$$
Since net charge inside cube is zero so net electric field entering should be equal to net electric field coming out should be equal
Thus , there should be 5 N/C electric field into the front face.