The equation to find the length of the fallen part of the tree  is 
[tex]x=\sqrt{8^2+13^2}[/tex]
Right Triangle. 
Right triangles is a triangle with one angle is 90 degree. 
The side opposite to 90 degree is the hypotenuse. 
Pythagorean Theorem 
 the sum of the squares on the legs of a right triangle is equal to the square on the hypotenuse .
The length of the fallen part of the tree is the hypotenuse 
Hypotenuse is 'x' 
Base of the triangle is 13 feet and height is 8 ft
Apply Pythagorean theorem to find out the hypotenuse 'x'
[tex]hypotenuse^2= base^2 + height ^2\\x^2=\sqrt{13^2+8^2} \\x=\sqrt{13^2+8^2}[/tex]
the equation to find the length of the fallen part is 
[tex]x=\sqrt{8^2+13^2}[/tex]
Learn more information about 'Pythagorean theorem ' here 
 brainly.com/question/22605943