Answer:
The new number would have to be 6.
Step-by-step explanation:
In order to find the mean you add all numbers and divide by how many numbers you added together. In this case you would add 3, 5, 7, and 9 and divide by 4 to get 6. If the median of the cards is still six you could multiply 6 and 5 because you have 5 numbers. Then subtract 9, 7, 5, and 3 which will give you six.