Okay, so what you need to figure out is that how many years in between 21 1/4 and 37 there are based on the amount the tree grows a year. So, 1 3/4 times 9 is 15.75, and this added to 21 1/4 is 37. This means that it will take the tree 9 years to grow from 21 1/4 ft to 37 ft.