In this question, you set up a ratio between the shadow lengths and another ratio for the heights and set them equal. The one quantity we don't know and which we are asked to find is the height of the tree.
Let T = the height of the tree
Then T/4.5 = 26/12 multiply both sides by 4.5 to isolate the variable T
T = (4.5)(26) / 12 = 9.75 feet