题目

 “Dad，” I say one day，“let's take a trip. Why don't you fly out and meet me？” My father had just retired after 27 years as a manager for IBM. His job filled his day, his thoughts, his life. While he woke up and took a warm shower, I screamed under a freezing waterfall in Peru. While he tied a tie and put on the same Swiss watch, I rowed a boat across Lake of the Ozarks. My father sees me drifting aimlessly, nothing to show for my 33 years but a passport full of funny stamps. He wants me to settle down, but now I want him to find an adventure. He agrees to travel with me在△ABC中，∠B=60°，∠BAC=75°，BC边上的高AD=3，则BC=    ．