题目

Mrs. White has a son. His name is Sam. The young man began to work in a post office in a big city this autumn. He asked his mother to visit the city. The old woman was happy and caught the train to the city, but she didn't know where the post office was. She began to look for it. At a bus stop she asked an old man, Excuse me, sir. How can I get to the post office, please?We have about forty post offices in our city, said the old man, Which one do you want to go to? The old woman gave Sam's address to him and the old man said, Walk along this street and take the fourth turning on the left. His 16．已知一次函数y=f（x）在区间[-2，6]上的平均变化率为2，且函数图象过点（0，2），试求此一次函数的表达式．