Answer:
Brand X has to mix 8.4 oz of brand A mixed nuts which contain 35% peanuts and 12.6 oz of brand B mixed nuts which contain 25%, to obtain 21 oz. Bags of mixed nuts that contain 29% peanuts.
Explanation:
Hi
We define
, wich means Brand A has 1 oz containing 35% peanuts.
, wich means Brand b has 1 oz containing 25% peanuts.
So we can build an equiation system
(1)

(2)
, after fixing it a little
(2)

As we can use any method to solve the system, I used a calculator wich thrown the following results
and
.