Complex roots of $z^2+3z+12$, $z^4+5z^2+4$, $z^6=1$ and $z^3=-1$

Find the (possibly complex) roots of these polynomials

\[ z ^ { 2 } + 3 z + 1 2 = 0 , \qquad z ^ { 4 } + 5 z ^ { 2 } + 4 = 0 , \qquad z ^ { 6 } = 1 , \qquad z ^ { 3 } = - 1 . \]