HOW TO FIND ANGLE - UNDERCUT -Slide Unit

Hi good People,

I am Tool Designer but I still have some questions in this area but no one know to explain me in my home country, that is reason why I am on the Forum.
I have one year experience with Tool Design Plastic as Intern(I learned alone and also with program Siemens NX in Company).
Now I don't work anymore but I want to know more about Tool Design myself from forum as I don't have chance to ask anyone to explain me what I want to know and also I like this area so much.
I hope so you guys can help me.

My question is :
How do I know angle here on this Indercut? Slide Unit(photo1)

On (Photo2) are correct angle of this Undercut and all other dimensions (Photo3) , (Photo4) and (Photo5)

This Undercut is from one Automotive plastic part and I want to understand exactly why is that 17 degree and all other dimension

I didn't have Trigonometry in the school/on the college but I have found on youtube useful about basic Trigonometry to understand angle.
Still I don't know how is here 17 degree (Photo2) why?
If you can explain me I would appreciate so much.

Thanks you and I wish you guys all the best.