We have twenty minutes' rest between classes. Look! Most of us are playing during the break time. Some students are on the playground. They are playing basketball. Oh! A boy is running with the ball. And another is trying to stop(阻止) him. They look so cool. And there are some girls watching the game. Some students are in the classroom. They are talking. A few of them are reading and doing homework. Look!  A girl is looking at the birds in the tree in front of the classroom. She must be thinking of something interesting because she is smiling. What are the teachers doing? Some of them are working in the office. And some are talking with students. Everyone is doing his or her things. They are busy but happy.

如图，已知AB∥CD∥EF，AD：AF=3：5，BE=10，那么BC的长等于____．

若4x²+2kx+9是完全平方式，则常数k=____．