By the problem the following gaseous reaction is occuring in closed, rigid vessel at constant temperature.
The stoichiometric ratio of number of moles of reactants and prducts is
The volume and temperature remaining constant the partial pressures of the component gases are proportional to their number of moles.
The initial partial pressures of
And the initial partial pressures of
If the partial pressure of
So the decrease in pressure of
But pressures of each of