Every year the tree grows 2 feet. I got this from the graph where is shows at 10 years the tree is 20 feet. I reduced both of the numbers by 10 to get 2 feet per year.
Knowing this we can use the equation y=2x.
We already know 100/2 is 50. So if we plug in 50 to our function. y=2(50)= 100.
So the tree will be 100 feet after 50 years.
Hope this helped!