VERY URGENT! What should be the statement for the Step 4 of Manuel’s Proof?

Which expression is equivalent to the expression shown?

If the value of cos330°is√3/2 then find cos 165°.

You're locked out of your house. the only open window is on the second floor, 25 feet above the ground. there are bushes along the edge of the house, so you will need to place the ladder 10 feet from the house. what length ladder do you need to reach the window?