Arrangements and Combinations question!

How do I do part 3?
I thought my method would work but it gives the wrong answer:

I took 600 as total number of possible combinations without the restrictions (from part 1), then took away the number of combinations that had both boiled rice and potatoes (3C1 x 6C3 x 4C4 = 60), giving me 540. Can someone explain what I did wrong please?

Thanks in advance!

Method was fine, but calculation in error.
Once you've chosen potatoes, then you need to choose another 3 (not 4) from the remaining four vegetables dishes, so:

3C1 x 6C3 x 4C3 = 240

Giving 600-240=360