You get the largest number if you sort the digits, from largest to smallest.
For this type of "the largest number which..." questions, you need to advance from left to right, using the largest possible digit in each case. For the first two digits, that would be 9, for the third digit (the right-most digit), the largest digit which will make this possible is an 8.
874321 is the largest number made from all these digits.
Pressumably, you want to use each digit once. Starting from the left, use the largest digit you can every time. For example, the left-most digit should be "6".
If you mean only using nines then 99.