Answer:
x - 231 ≤ 459
Step-by-step explanation:
Given:
Elevation ranges below sea level  = 228 ft
Elevation ranges above sea level  = 690 ft
Elevation = x
Computation:
Ideal range = [690-228] / 2 = 231
Tolerance range = [690+228] / 2 = 459
So,
x - 231 ≤ 459