题目

When I was a kid in Minnesota, watermelon was expensive. One of my father’s friends, Bernie, was a rich businessman, who owned a large store in St. Paul.Every summer, when the first watermelons arrived, Bernie would call. Dad and I would go to Bernie’s place. We’d sit on the edge of the dock(码头), feet dangling(摇晃), and got ready for a big meal.Bernie would take his knife, cut our first watermelon, hand us both a big piece and sit down next to us. Then we’d bury our faces in watermelon, eating only the heart—the reddest, juiciest, firmest, sweetest, most seed-free part6．若函数f（x）是一次函数，且f（f（x））=4x+1，则f（x）=$2x+\frac{1}{3}，或-2x-1$．