Answer
B
Work Step by Step
Notice that graphite, as a solid, isn't factored in the equilibrium.
For CO2, the reacted concentration being x, leads to, by stoichiometry, a final concentration of CO of 2x.
$K=[CO]^2/[CO_2]$
$0.021=(2x)^2/(0.012-x)$
$0.000252-0.021x=4x^2\rightarrow 4x^2+0.021x-0.000252=0$
$x=+0.0057\ or\ -0.011\rightarrow x=0.0057\ M$
The equilibrium concentration of CO is 2x: $0.0114\ M$
