First, we are discussing reversible reactions here, since the reaction is proceeded in both directions forming both reactants and products.
The reaction is symbolized as:
A + B <.........> C + D
The first given is:
The rate of appearance of D is twice the disappearance of A
This means that the coefficient of D in the reaction is twice the coefficient of A
The second given is:
The rate of appearance of D is one third the disappearance of B.
This means that the coefficient of D in the reaction is 1/3 that of B. This also means that the coefficient of B is 3 times that of D.
Combining these two pieces of information, we will find that the best equation that resembles this scenario is:
A + 6 B <.........> C + 2 D
