How to answer this problem and whats the answer

Evaluate cos2θ for cosθ = square root of 3/2

Use the elimination method to solve the systems of equations.choose the correct ordered pair. 7x+4y=39

Answer asap and if you do you will get brainliest. catherine buys a gallon of ice cream from the store. after taking it home, she eats a fifth of a gallon of ice cream. her sister eats some of the ice cream as well. if two-thirds of the original amount of ice cream is left, then what fraction of a gallon of ice cream did her sister eat?