Holly's favorite book is about Johnny Appleseed, the American pioneer who planted apple trees all across the country. Inspired by the story, Holly plants an apple seed in her backyard and tends the seed as it slowly grows into a tree. The proportional relationship between the age of Holly's apple tree (in years), x, and the height of the tree (in feet), y, can be modeled by the equation y = 1.5x.If the tree is 5.25 feet tall, how many years has the tree grown?