answer
given
v=2100cm³
h=3r
v=1/3πr²h
soln
v=1/3πr²h
2100cm³=1/3×π×r²×3r( you will insert 3r in the place of h.)
2100cm³=3π/3×r³(cancel 3 by 3)
2100cm³= πr³
r³=2100cm³/3.14 ( π=3.14)
¾r³=¾2100cm³/3.14(I make them cube root.sorry I can't use the real cube root so I replace it by ¾)
r=8.74…