Answer
$\Phi = 0$
Work Step by Step
We can find the electric flux through the right face:
$\Phi = \int E \cdot dA$
$\Phi = [(-3.00~N/C)~\hat{i}+(4.00~N/C)~\hat{k}]~\cdot (1.40~m)^2\hat{j}$
$\Phi = (-5.88~N~m^2/C)\hat{i}\cdot \hat{j}+(7.84~N~m^2/C)\hat{k}\cdot \hat{j}$
$\Phi = 0 + 0$
$\Phi = 0$