Partial pressure problem?

Question

At 25°C, Kp = 5.3 105 atm-2 for the following reaction.

N2(g) + 3 H2(g) 2 NH3(g)
When a certain partial pressure of NH3(g) is put into an otherwise empty rigid vessel at 25°C, equilibrium is reached when 50.0% of the original ammonia has decomposed. What was the original partial pressure of ammonia before any decomposition occurred?

Answer 1

Let p equal the equlibrium pp of NH3. It must have started out being 2p.

Then there must be 1.5p of hydrogen in there, and 0.5p of nitrogen.

Solve p squared / 1.5 p cubed x 0.5p = 5.3 x 10(5) and then work out what 2p is.