In a undisturbed layer, how do we know which layer was deposited first ?

It's the bottom layer.  

Explanation:

Scientists use the Law of Superposition to determine the relative age of a layer of sedimentary rock:

The oldest rock layer is at the bottom of an undisturbed bed.

Thus, Layer A in the figure below was deposited first.

2h2o2--> 2h2o + o2

this reaction is of the spontaneous decomposition of hydrogen peroxide down into water and oxygen.

add 2 molecules of hydrogen peroxide and 2 molecules of water. since oxygen is naturally diatomic, the total number of atoms of each element is now the same on both sides of the equation so it is balanced.

this may not be entirely correct but let me know.

since electrons are negative the answer is neagtaive