Finally, we propose a framework to put constraints on the dark energy equation of state by using the joint likelihood of the X-ray AGN clustering and of the Hubble relation cosmological analyses. A preliminary joint analysis using the X-ray AGN clustering of the 2XMM survey and the Hubble relation of the Constitution SNIa set provide Omega(m) = 0.31 +/- 0.01 and w = -1.06 +/- 0.05. We also find that the joint SNIa-2XMM analysis provides significantly more stringent cosmological constraints, increasing the figure of merit by a factor of similar to 2, with respect to that of the joint SNIa-baryonic acoustic oscillation analysis.