Find the measure of the third side of the given triangle. P stands for perimeter and has the value 12ײ-7×+9. (Hint:the perimeter is the sum if the three sides of the triangle.) let S represent the measure of the third side. please answer this if u know the right answer​

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?