It is neither, it is a pure substance.
Synthesis, this is a synthesis reaction because it went from A+X=AX
Answer: 4.
and 
Explanation:
a) The given reaction is 
As the mass on both reactant and product side must be equal:


As the atomic number on both reactant and product side must be equal:



b) 
Total mass on reactant side = total mass on product side
15 =15 + x
x = 0
Total atomic number on reactant side = total atomic number on product side
8 = 7 + y
y = 1

By decreasing n we can increase presure because decrease in n will shift equilibrium to either forward or reverse direction