阅读短文，选择正确的答案。

    There are many apple trees in a garden. They're good friends. One day an old tree is ill. There are many pests in the tree. Leaves of the tree turn yellow. The old tree feels very sad and unwell. Other trees send for a doctor for him. At first, they send for a pigeon, but she has no idea about it. Then they send for an oriole(黄鹂), and she can't treat the old tree well either. Then they send for a woodpecker. She is a good doctor. She pecks a hole in the tree and eats lots of pests. At last the old tree becomes better and better. Leaves turn green and green.

等腰三角形的周长为30，腰长是12，则底边长为____.