Answer:
Middle: 2.5 ft
Right: 4,375 ft 
Left: 1,875 ft
Explanation:
We know that a tree in the middle is 2.5 feet tall. 
Now the tree on the left measures 3/4 of the tree in the middle, therefore: 
2.5 * 3/4 = 1.875 
Which means that the tree on the left is 1.875 feet tall. 
Finally the tree on the right 1 3/4, which would be 1.75 times the tree in the middle, therefore: 
2.5 * 1.75 = 4.375 
Therefore the tree on the right is 4.375 feet tall.