A cubical volume is bounded by the surfaces x = 0, x = a, y = 0, y...

Given here, electric field, \overset{\to}{E} = {E}_{0}x\overset{^}{i}

Here, the flux passes mainly through surface areas, $ABCDandEFGH$. As the surfaces $AEFBandCGHD$ are parallel to the electric field, so flux for these surfaces are zero. Again in $EFGH$, $a = 0$, thus, electric field is zero.

Hence, the flux only passes through the surface are $ABCD$.

The net flux is ${ϕ}_{net} = {ϕ}_{ABCD} = {E}_{0}a \cdot {a}^{2}$

Using Gauss's law, $\dfrac{{q}_{en}}{{ϵ}_{0}} = {E}_{0}{a}^{3}$

$⇒ {q}_{en} = {E}_{0}{ε}_{0}{a}^{3}$

$= 4 \times {10}^{4} \times 9 \times {10}^{- 12} \times 8 \times {10}^{- 6}$

$= 288 \times {10}^{- 14}C$

Hence, the value of $Q = 288$.