For this problem you need to use the Pythagorean theorem. The adjacent side to the right angle is the 4 feet and the opposite side is 7 feet.
4^2 + 7^2 = x^2
16 + 49 = x^2
65 = x^2 => 8.06225... So the distance is 8.1 feet to from the top of her head to the shadow.
