Finding equilibrium concentrations in solution from initial reagent concentrations and equilibrium constant
No need for the cubic equation. Just firstly assume that $\ce{A}$ and $\ce{B}$ reacted completely, then $\ce{[C]}=\pu{2M}$, $\ce{[D]}=\pu{1M}$, and from $K_c$ you can calculate $\ce{A}$ which is the same as $\ce{B}$.
That's the approximation usually used with this kind of problem.
The answer should be $\pu{2E-4M}$.