You can use the pythagorean theorem to find the missing side length of this triangle.
a² + b² = c²
Now, just plug in the numbers.
a² + 5² = 80²
So, a² + 25 = 6400
Now, just perform the order of operations on both sides of the equation.
a² + 25 = 6400
-25 -25
------ -------
a² = 6375
√a² = √6375
a = 79.8 meters, if the hypotenuse, or slant height of the boy to the tree is 80m.