$B + C \rightleftharpoons P \quad K _{ eq }^{(2)}=\frac{[ P ]}{[ B ][ C ]}$$.....(2)$
For
$A \rightleftharpoons P \quad K_{e q}=\frac{[P]}{[A]}$
Multiplying equation $(1)$ $\&$ $(2)$
$K _{ eq }^{( l )} \times K _{ eq }^{(2)}=\frac{[ P ]}{[ A ]}= K _{ eq }$
$Fe _{2} N ( s )+\frac{3}{2} H _{2}( g ) \rightleftharpoons 2 Fe ( s )+ NH _{3}( g )$