In this case, we'll have to carry out several steps to find the solution.
Step 01:
Data
airport altitude = 750 feet above sea level
plane altitude = 1600 feet above sea level
sea level = ?
Step 02:
Sea level:
Sea level is taken as a reference for altitude, therefore its value is zero.
750 - 750 = sea level
0 = sea level
1600 - 1600 = sea level
0 = sea level
The answer is:
sea level = 0