((Please help)) A tree casts a 26-ft shadow. A boy standing nearby casts a 12-ft shadow, forming similar triangles. His height is 4 ft. How tall is the tree?

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