  Characterization of the multipartite mixed state entanglement is still a
challenging problem. Since due to the fact that the entanglement for the mixed
states, in general, is defined by a convex-roof extension. That is the
entanglement measure of a mixed state \r{ho} of a quantum system can be defined
as the minimum average entanglement of an ensemble of pure states. In this
Letter, we give an explicit formula for the polynomial entanglement measures of
degree 2 of even-N qubits mixed states that is similar to Wooters formula in
[1]. Then we discuss our findings in the framework of X density matrices and
show that our formula for this type of density matrices is in the full
agreement with the genuine multipartite (GM) entanglement of these states.
