Ab Surd Ity
For the first part try squaring the whole expression.
For the second part, you could write $a = {\root 3\of
{2+\sqrt{5}}}$ and $b= {\root 3\of {2-\sqrt{5}}}$, and use the
identity $$(a + b)^3 = a^3 + 3a^2b + 3ab^2 + b^3 = a^3 + 3ab(a + b)
+ b^3.$$