Do not just read the derivations in the PDF. Copy them by hand. Chopra & Agarwal lay out derivations step-by-step. Write each step on paper. EM theory is visual and mathematical; handwriting encodes it into your muscle memory for exams.
Simply downloading a PDF and glancing at it will not help you pass. Here is a strategic study plan: electromagnetic theory chopra agarwal pdf
Solution: Use Gauss’s Law (Total flux = Q/ε0). By symmetry, flux through one face = Q/(6ε0). The book has a diagram for this. Do not just read the derivations in the PDF