His sister is 4 feet tall. her shadow is 7 feet long. from the top of her head to her shadow, how long is it? (nearest tenth of a foot)

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. 

The right answer should be 6 but your not very specific