For question 17, I think you've understood quite well what you're doing. Just stick a zero in for the initial concentration of nitrogen gas, since it doesn't say anything about nitrogen at the beginning of the question. The assumption is that if it isn't mentioned, it doesnt exist at the start. Because what's given is added to an empty vessel, nothing was in the vessel to begin with and nothing more or less than what is added is added. Crunch the numbers and you should get that
For 19, I think you should use an ICE table. What you've done there is add some ammonia that doesnt exist by subbing directly into the equilibrium constant equation, and that doesnt work. Using an ICE table, you eventually get to an expression like this:
I'm going to trust your maths skills here, but if you need any help with this part ask again
. Solve for x, which is roughly 8.28 by 10 to the -3. Since the equilibrium constant is zero at the start (since ammonia has zero concentration at the start), and now it has a value, then it follows that equilibrium shifts right.
For 20, a) is pretty much spot on. At the start, Q is 150, so at equilibrium, there will be more reactants and less products. So that part's great
With b) you have to notice what's implied by this. As the temperature increases, there are more products (implied by increase in constant). This tells you that when you increase the temperature, by LCP the system shifts right to minimise the change, which would be to absorb it. Therefore the forward reaction is endothermic, and therefore the enthalpy change is positive.
Hope this helps