The tree is already 48 inches tall. We add on 3x to represent the idea the tree is growing 3 inches per month. Therefore, the total height is 48+3x or 3x+48 inches.
Let's say x = 5 months have elapsed as an example.
The tree would have grown 3*x = 3*5 = 15 inches to get 48+15 = 63 inches tall. In other words, y = 3x+48 will have y = 63 when x = 5.
