Atree casts a shadow 21 feet long, while enzo, who is 5 feet tall, casts a shadow 6 feet long. what is the height of the tree?

20 feet long because you just minus 1 to whatever the shadow is 

Basically, you would first like to draw a picture of the problem.

then, after you do that, you will want to put them in ratios. here's what I did:
6/5=21/x
since we want to solve for x, the height of the tree, we will reform the equation for x to be alone(you will want to do cross multiplication here):
6x=5*21
6x=105
x=17.5