Consider a closed triangular box resting within a horizontal electric field of magnitude E = 6.74 10^{4} N/C as shown in the figure below.

The formula for electric flux is as follows:

Electric flux = E*A (* E = electric field, A = area)

and if electric flux is at an angle from the normal (which is perpendicular) the formula is:

Electric flux = E*A*cos(theta) (* E = electric field, A = area, theta = angle between the normal of E and A)

This area is equal to,

A = length * width = 30cm * width.

A = 30cm * (10cm / cos(60)) = 600 cm^2 = 0.06 m^2

Plugging into the formula we get:

Electric flux = (6.74 104 N/C) * (0.06 m^2 )*cos(60) = 2022 N · m2/C

Electric flux = 2022 N · m2/C = 2.022 kN · m2/C

Part C, the elctric flux through the entire surface would just be the sum of part A and B,

whic is - 2.022 kN · m2/C + 2.022 kN · m2/C = 0

so electric flux = 0