Looking at the picture it is just a triangle with a hypotenuse of 10 (the ladder) and the bottom side of 6 (distance from the ladder to tree). All you have to do now is the Pythagorean theorem: a^2 + b^2 = c^2
10^2 - 6^2 = b^2
100 - 36 = b^2
64 = B^2 now square root both sides.
Answer is C. 8 feet