The area of any triangle = (1/2) x (length of the base) x (height)
2.5 centimeters = 0.025 meter
The area of the sail is (3.75) = (0.5) x (0.025) x (height)
Multiply each side by 2 : (7.5) = (0.025) x (height)
Divide each side by 0.025 : (7.5)/(0.025) = height = 300 meters
The situation is stupid, but the math is bullet-proof.