The fact that an oncoming plane passed is irrelevant to what we need to find, so we can ignore that.
The math statement it's asking you to solve is this:
0+21000-(4329+8543)
The reason we are adding 4329 and 8543 is to make it easier on the subtraction. Since both of those terms are being subtracted, we can combine them to make our problem one step shorter.
After simplifying the parentheses, we get:
21000-12872
After this it becomes simple subtraction to find the answer.
8128 is the answer, so your answer would be:
The plane is now flying at an altitude of 8,128 feet.
