根据短文理解，完成下列各题。

    Helen and Linda are good friends. They are in the same class. They go to school five days a week. And they have seven classes a day. After school they play volleyball.

    The two girls' mothers are friends, too. They like sports. So they can play sports with the two girls on the weekend（在周末）. Linda's mother likes tennis and Helen's mother likes tennis, too. Can the two girls play tennis? Sure, they play it well.

    The four friends always play tennis in the morning, and watch TV in the afternoon. It's relaxing! The two girls like eating fruits. They like apples, strawberries and oranges. They think fruits are good and healthy. After playing tennis, their mothers always make (做) some fruit salad first. What good friends they are!

    Do you like Sun Wukong? He is one of the most popular novel characters in Chinese history. Not only young children but also adults love him so much. In our mind, he is a great hero in the Chinese novel Journey to the West.

    In the novel, Sun Wukong, Zhu Bajie and Sha Seng beat demons(妖魔) and protect Tang Seng when they travel to the Western Paradise(西天). Sun Wukong was born from a stone. He can change himself into a tree, a bird, or and animal. With one somersault(筋斗), he can travel 108,000li.

    Sun Wukong is clever, brave and always makes fun magic tricks. Chinese people love him very much.

    Sun Wukong is also popular in other countries. Many western children love him and learn kung fu because of him. To them, he is a super hero, just like Superman and Spiderman.

等腰三角形的两条边长分别为3，6，那么它的周长为（　　）