HW HELP

My friend was supposed to help today during lunch but we got distracted by a game of Bullshit, so if anyone here can do this stuff, I’d really appreciate some help.

We’re doing Law of Cos and here is a pic of the problem i have to solve:

picture 3 - hw help

I know I have to use the Side-Side-Side formula, which is: a^2-b^2-c^2 = -2bc(cosA)

Once everything is plugged in and switched around to isolate A, that formula turns into:
6^2 + 8^2 + 12^2 / 2(8)(12) = cosA

After that, to isolate A from cos, you need to use the inverse (divide both sides by cos) but I don’t quite understand the whole inverse thing. I thought the inverse was when it was like cos^-1 but every time I plug that into my calculator, it says error and I don’t know what I’m doing wrong.