You need to find the oxidation states of each element in the compound.
To do that you base your work in two steps:
1) State the oxidation states that can only work with one state
2) Calcualte the missing oxidation states needed to meet the final net of the compound (neutral compounds have zero net charge).
P2X3: phosphuros can work with oxidation states 3+ and 5+
Then the two options are:
2(3+) + 3(x) = 0 => X = - (6+) / 3 = -(2) = (2-) and
2(5+) + 3(x) = - 10/3.
Then you choose the integer value (2-).
And the two oxidation states are 3+ por phosphorus and 2 - for X.
P2X5,
That is 2(5+) + 5(x) = 0 => x = - 2(5+) / 5 = -2
Then the two oxidation states are 5 for phosphorus and 2- for X.
H2X2,
H oxydation state is 1+ => 2(1+) + 2(x) = 0 => x = -1
Then the oxydation state for X is 1-.
H2X,
H: 1+
=> 2(1+) + x = 0 => x = -2.
=> Oxydation state for X = 2 -
CX2,
C: may have 2 or 4, so
option 1: (2+) + 2(x) => x = -1
option 2: (4+) + 2(x) = 0 => x = -4/2 = - 2,
You cannot know which is the final oxydation state or less you are told that the structural formula is given by CX2 rather than C2X4.
If CX2 represents the structural formula, then you can tell that the oxydation state is given by the option 1 and the oxydation state of X is 1-.
CX
C may have oxydation states 2+ or 4+, then X may also have any of those two oxydation states, unless other information about the structure of the product is given.
