Answer: The height of fir tree is 9 meters and the height of pine tree is 12 meters.
Explanation:
Given : The combined height of one fir tree and one pine tree = 21 meters.
Since, the height of 4 fir trees stacked on top of each other is 24 meters taller than one pine tree.
Let x be the height of fir tree and y be the height of pine tree.
Then we get the following equations:

Adding (1) and (2) we get

Substitute the value of x in (1), we get

Hence, the height of fir tree is 9 meters and the height of pine tree is 12 meters.