Question 363806
{{{x^(2/3) - x^(1/3) - 20 = 0}}},
{{{(x^(1/3)-5)(x^(1/3)+4) = 0}}},
{{{x^(1/3)-5= 0}}} or {{{x^(1/3)+4 = 0}}}.  Thus {{{x = 5^3 = 125}}}, or {{{x = (-4)^3 = -64}}}.
To check:
{{{(-64)^(2/3)-(-64)^(1/3) - 20 = (-4)^2 - (-4)-20 = 16+4-20 = 0}}}, and 
{{{(125)^(2/3)-(125)^(1/3) - 20 = 5^2 - 5-20 = 25-5-20 = 0}}}.