Hi, I am just a little bit stuck on a question. Chemistry

Hi i am doing chemistry 12 equilibrium but i got stuck on a question while studying
i am trying to do it but nothing comes to mind

so it says

consider the system 2hbr <--> h2 + br2
initially .25 mol of h2 and .25 of br are in a 500 ml vessel that is heated . k for the reaction is .020

calculate the concentrations.
calculate the amount in moles of each substances present at equilibrium
calculate the extent of reaction as a percent reaction

i know percent reaction is theoretical\actual yield i think

and i converted 500 ml to L
so i got 0.5 L

and i found the initial concentration of h2 and br2
they are both o.5 mol/L i just took the mol that they were initially at and divided by .5L

but then i got stuck on the next 3 questions. how would i find all concentrations and then the moles and then % yield? :S

May someone please help..

[ ] means concentration

at equilibrium...
k = [H2] x [Br2] / [HBr]² .. by definition of k

because...
1 mole H2 + 1 mole Br2 ---> 2 moles HBr..
and because both started out at 0.25 moles / 0.5L...
either H2 or Br2 could be considered "limiting" the reverse reaction

so if we let.. "2x" = moles HBr at equilibrium.. then 1x moles H2 and 1x moles Br2 have been consumed...
ie.. concentrations at equilibrium are
[H2] = (0.25 - x) / 0.5L
[Br2] = (0.25 - x) / 0.5L
[HBr] = 2x / 0.5L

Capice?

*************
so...
k = [H2] x [Br2] / [HBr]² = ((0.25 - x)/0.5L) x ((0.25 - x)/0.5L) / ((2x)/0.5L)²
k = (0.25 - x)² / (2x)²
k = (0.25 - x)² / (4x²)

and we know that k = 0.020.. so that...
0.020 = (0.25 - x)² / (4x²)

solving for x... along with a bit of manipulation..
1/50 = (0.25 - x)² / (4x²)
4/50 x² = (0.25 - x)²
4/50 x² = 1/16 - 1/2x + x²
64/50 x² = 1 - 8x + 16x²
64 x² = 50 - 400x + 800x²
736x² - 400x +50 = 0

and using the quadratic equation
x = [400 ±√(400²-4*736*50)] / (2*736)
x = (400 ± 113.14) / 1472
x = 0.348 and x = 0.195

now.. the x = 0.348 doesn't work because the concentration of H2 and Br2 < 0.. right?
[H2] = (0.25 - 0.348) = (-0.099)

so.. x = 0.195..
and [H2] = [Br2] = 0.25 - 0.195 = 0.05 / 0.5L = 0.10 moles / L = 0.10M
and [HBr] = 2x0.195 = 0.39 / 0.5L = 0.78 moles / L = 0.78M

************
So.. for your answers....

1).. initially.. [H2] = [Br2] = 0.25moles / 0.5L = 0.5M
2).. at equilibrium.. [H2] = [Br2] = 0.10M and [HBr] = 0.78M
3).. extent of reaction = moles H2 present / moles H2 possible = 0.10M / 0.50M = 20%