Consider the following reaction
A+B⇌E;Kc=6......(1)
2B+C⇌2D;Kc=4.....(2)
The equilibrium constant (Kc) for the following reaction will be 3
A+D⇌E+12C
Reverse reaction (2)
2D⇌2B+C;Kc=14=0.25.....(4)
Divide the reaction (4) with 2
D⇌B+12C;Kc=√0.25=0.5.....(5)
Add reactions (1) and (5)
A+D⇌E+12C;Kc=6×0.5=3