The square root of 36 is 6, because 36=6*6. (Radical Sign)36=6, never be -6. The radical sign always means the positive square root, so that (-)(Radical Sign)36=-6. This rule(?) may be an axiom sort of I guess.
The square roots of 36 are +6 and -6, because 36=-6*-6 as well.
And don't say "the cube roots of 27 are ..." in plural form, because the cube root of 27 is 3 only, never be -3.