A and B react as follows: A + B eq 2C. The equilibrium constant is 5.0 x 10^6.?

________________ A ______ + ______ B ______ ⇌ _____ C ______ Kc = 5.0 × 10⁶
Initial: _____ 0.4 mol/L _________ 0.7 mol/L________ 0 mol/L
Change: ____ -y mol/L __________ -y mol/L _______ +2y mol/L
At eqm: __ (0.4 - y) mol/L _____ (0.7 - y) mol/L _____ 2y mol/L

As Kc is very great, A is almost completely converted to C.
Hence, (0.4 - y) ≈ 0 and thus y ≈ 0.4

At equilibrium :
[B] ≈ (0.7 - 0.4) mol/L = 0.3 mol/L
[C] ≈ 2 × 0.4 mol/L = 0.8 mol/L

Kc = [C]² / ([A] [B])
5.0 × 10⁶ = (0.8)² / ([A] × 0.3)
[A] = (0.8)² / {(5.0 × 10⁶) × 0.3} mol/L
[A] = 4.3 × 10⁻⁷ mol/L

Hence, at equilibrium
[A] = 4.3 × 10⁻⁷ mol/L
[B] = 0.3 mol/L
[C] = 0.8 mol/L