Could someone please explain to me why the formula that is being used is being used in part c particularly (as in explaining the formula)?

To be clear, I am talking about the (r-r_0) * [(r_1 -r_0) x (r_2 -r_0)] =0formula. If you're unsure of what I am asking or need more information or something, just ask.

Sorry if this is not the right place to post but I don't know where else to post it since it's in my calculus course.

Any input would be appreciated!

Thanks in advance!