… using equilibrium in a specific cross-section#

This is treated in chapter 14.1.1 of the book Engineering Mechanics Volume 1 (Hartsuijker and Welleman, 2006).
Exercises#
Exercises in chapter 14.4 of the book Engineering Mechanics Volume 1 (Hartsuijker and Welleman, 2006), answers are available on this website: 14.1 - 14.49