Paper 1 October/November 2017 CHEMISTRY (9701/11) IGCSE

Gas X dissociates on heating to set up the following equilibrium.
X(g) $\rightleftharpoons$ Y(g) + Z(g)
A quantity of gas X is heated at constant pressure, p, at a certain temperature.
The equilibrium partial pressure of gas X is found to be $\frac{1}{7}$p.
What is the equilibrium constant, K$_p$, at this temperature?