A10ft flagpole stands next to a tree. the shadow of the flagpole is 17ft and the shadow of the tree is 50ft. what is the height of the tree?

Will solve this problem using something called, "proportions."
Where one thing is proportional to another thing. 

While this isn't always the case, most of the time the two similar measurements they gave you are the measurements you'll be using as your proportion. This is true in this particular case. 

50/17 = x/10

Cross multiply!

17x = 500

Solve for x!

x = 500/17 = 29.41

Rounded off the tree is: 
30 ft tall!(Or, 29 ft tall depending on how you want to round it)