The setup for 3a looks good though I didn't double-check the calculations; one thing to add is that you could save some time by excluding the 100 L value from the calculations since you are only concerned with the number of moles of HI, which is all you need to find the grams of HI for the answer.
For 3bi, you should have K1K2 = [HCl]^2/[H2][Cl2]
find the moles of H2 gas at equilibrium from 3a - you can do this, so I'll just call it A moles; then you should have:
So for 3bii, it's 2K1^2 = x^2/(A - 0.5x)(40/71) -- you know K1 from before (6.865), so solve for x to get the answer.
