I don't understand what to do. It says use a calculator when necessary.
First, make sure your calculator is in "radian" mode, not degrees. Your calculator has an "inverse" button and a "cosine" button. Press "inverse", "cosine", then enter the number and "return". That will give you the "principal value, between and .
Because cosine is an "even function", cos(x)= cos(-x), the negative of that value is a solution also- that's why they say "plus or minus". And cosine is periodic with period . That's why adding " " for any integer n also is an answer.