When you get problems like this, its great to start with a picture. I can't draw it here, but if you draw a giant circle, you can see that the diameter (length from one side of the circle to the other) in the middle of the circle is a LOT bigger than the length of a line drawn higher up on the circle from side to side. This is the same way Jupiter looks (sort of :^/)
If the middle line is 88,000 miles long (please don't draw a 88,000 mile long line) and the higher line is 6,000 less miles. All you have to do is subtract 6,000 from 88,000 and you will find out how long the higher line is.
So that's all, we subtract the 2 numbers and we find the polar diameter! Hope that helped, Emily