You have to have a better question to answer that, 100 feet more what?
Volume is Length x width x height. So what are three numbers that multiply to 90?
30x30x30=90
The formula for finding the length is

All we have to do is plug in the numbers.

36 + 64 = 100
Since 100 =

we have to find the square root.

The length of the slide is 10 feet.
The standard equation of a circle is:

where (h,k) is the center and r is the radius.
We need to make an equation of a circle, whose center is (11,-5) with any radius. Therefore, substitute the value.

The bolded part is your answer. Since the radius is not given, we can put any value.