X is 4 because 64 is 8 squared, so 4 would be 2 squared
Greetings!
Yes, the degree on a monomial can be negative:
Example:

This is equal to:

Hope this helps.
-Benjamin
Volume equals to 1539.38 cm^3
The missing factor is (a -1) I believe. If you foil (5a + 9)(a - 1) you would get 5a^2 + 4a - 9.