The bases form a square. The 123.7 ft dimension would be a diagonal dimension of the square.
Find the side of the square using the formula Side = √2 * d/2 where d is the diagonal.
Side = √2 * 127.3/2 = 90.014 feet, round to 90 feet.
Use the Pythagorean theorem:
X = √(20^2 - 10^2)
X = √(400 - 100)
X = √300
X = 17.32 meters
Round to 17 meters.