I had trouble with these concepts as well. You don't have enough info in the question to answer directly.
Using Dalton's law total pressure = sum of all the partial pressures. A partial pressure is defined as if the 1 item is in the system all by itself..
P total= P a + P b + P c.... P z
If you know the Temperature,Volume and number of moles of each item you can then figure out the partial pressures.
Each unknown makes the problem tougher but not impossible to solve. Also keep track of the changes to your system.
If A + B = C A and B may be reduced and C is increased (larger molecule but fewer total indvidual molecules?). Heat may also cause some complications Often constant T is defined as part of the puzzle.