Given the formula for volume, solve for diameter:

V= (4/3) pi * r^3
36 pi = (4/3) pi * r^3
Divide both sides by pi
36 = (4/3) * r^3
Multiply both sides by the reciprocal of (4/3) which is (3/4).
36(3/4) = (3/4)(4/3)* r*3
27 = r^3
Take the cube root of both sides
3 = r
3 cm