Please download the free trial version of Algebrator for this problem, press the Wizard button and choose the wizard titled "Slope of the line joining two points"... press "Solve Step" and then "Explain" to see a detailed explanation of how to solve this problem.

Windows: https://www.softmath.com/tr/alg51win-tr-en.exe

Mac: https://www.softmath.com/tr/alg51mac-tr-en.dmg