Hays' body and his shadow are perpendicular.
So if you imagine the line from the top of his head to the end of his shadow on the ground, you have a right triangle, and the imaginary line is the hypotenuse. Then you can stand back and let Dr. Pythagoras figure out the length of the line for you, using c² = a² + b²
(Distance)² = (Hay's height)² + (length of his shadow)²
(Distance)² = (6 ft)² + (4 ft)²
(Distance)² = (36 ft²) + (16 ft²)
(Distance) = 52 ft²
Distance = √(52 ft)²
Distance = 7.21 ft