Let (a+bx+cx²)10 = $ \sum_{i=0}^{20} $ pixi, a,b,c∈N. If p1=20 and P₂ = 210, then 2(a+b+c) is equal to

What will be the equilibrium constant of the given reaction carried out in a \(5 \,L\) vessel and having equilibrium amounts of \(A_2\) and \(A\) as \(0.5\) mole and \(2 \times 10^{-6}\) mole respectively?
The reaction : \(A_2 \rightleftharpoons 2A\)