The answer is 3√5 mi.
The formula is: d = √(3h/2)
Wyatt:
h = 120 ft
d = √(3 * 120/2) = √180 = √(36 * 5) = √36 * √5 = 6√5 mi
Shawn:
h = 270 ft
d = √(3 * 270/2) = √405 = √(81 * 5) = √81 * √5 = 9√5 mi
How much farther can Shawn see to the horizon?
Shawn - Wyatt = 9√5 - 6√5 = 3√5 mi