Check if i got the right answer on 3a, . only 3a

This is how you should be doing,
P reacts with excess Cl₂ 
2P + 3Cl₂ > 2PCl₃
Cl₂ is excess therefore amount of PCl₃ formed will depend on P.
 
According to stoichiometry,
2 moles of P gives = 2 moles of PCl₃
∴ 5 moles of P will give = 5*2/2 = 5 moles of PCl₃
You are correct.

For any point (x, y), that appears in one function, the point (y, x) must appear in the other function.  

For any point (x, y), that DOESN'T appear in one function, the point (y, x) must NOT appear in the other function.  

So if you find any point that DOESN'T meet the above rules, you can prove that the functions are NOT inverses of each other.  

But if you want to prove that they ARE inverses of each other, you have to check ALL points in the functions, and there are infinity of them, so that's not practical. Instead, you can write f(x) like this:  

y = 3x - 5  

Now solve for x:  

y + 5 = 3x  

x = (y + 5) / 3  

Now swap x and y:  

y = (x + 5) / 3  

This is the same as g(x). Therefore the functions are inverses of each other.

Step-by-step explanation: