ive tried the ice method but i keep on getting it wrong please help.

If the initial pressure of NH3(g) is 3.168 atm, calculate the % decomposition of NH3(g) when the reaction comes to equilibrium according to the balanced equation. The value of Kp at 427.0 °C is 12800.00. The initial pressure of the reaction products is 0 atm.

2NH3(g) = N2(g)+3H2(g)

Instead of me working the problem for you and you checking my work, why not show your work and let us check your work. Be happy to do that for you.