Why don't you show your working so we can see whether you're insane or not?
Yes, you do get a cubic equation. But in this case you can use the fact that Kc is very small to predict that the equilibrium is well over to the left. For part a, using the ICE table, I get
Kc = 4x3/(1-2x)2, where x is the equilibrium concentration of Cl2. Is this what you get?
But since K is so small, we can try making the simplification that x << 1, so (1-2x) ≈ 1. Then it's easy to solve, and you can (should) then check whether the assumption x << 1 is true.