Determine Cubic Equation grade 12

Determine Cubic Equation grade 12

[theboy]

Thanks for the videos they are really helpful.
I'm a bit confused because when you are finding the inflection point you are finding the x coordinate for when the graph changes from concave up to concave down and I understand why the second derivative was 0 but I don't understand why the x coordinate of the second derivative was also 0. I hope you understand my question

[KgalabiMoabelo]

I don't Understand so much in this video, i hope it doesn't come out in the exam tomorrow