Question 57756
Hint:
When writing formulas containing duplicate copies of the same letter of the alphabet, such as your formula for the volume, {{{V = (4/3)(pi)r^2 - (4/3)(pi)r^2}}} you must use upper and lower case letters to distinguish between the two variables, otherwise, one cannot make much sense out of the formula.  Her's how it should appear:
{{{V = (4/3)(pi)R^3 - (4/3)(pi)r^3}}} where: R is the outside radius and r is the inside radius.

a) Rewrite by factoring the right sides.

{{{V = (4/3)(pi)(R^3 - r^3)}}}

b) I don't see your graph but if I graph {{{V = (4/3)(pi)(R^3 - 27)}}}...the 27 comes from inside radius of r = 3 cm but it's {{{r^3}}} in the formula:{{{r^3 = 3^3}}} = {{{27}}}

{{{graph(300,200,-5,5,-120,130,(4/3)(3.14)(x^3-27))}}}

It's a little hard to tell from this graph but when V = 100, then R = 3.7 cm