  Current efforts in observational cosmology are focused on characterizing the
mass-energy content of the Universe. We present results from a geometric test
based on strong lensing in galaxy clusters. Based on Hubble Space Telescope
images and extensive ground-based spectroscopic follow-up of the massive galaxy
cluster Abell 1689, we used a parametric model to simultaneously constrain the
cluster mass distribution and dark energy equation of state. Combining our
cosmological constraints with those from X-ray clusters and the Wilkinson
Microwave Anisotropy Probe 5-year data gives {\Omega}m = 0.25 +/- 0.05 and wx =
-0.97 +/- 0.07 which are consistent with results from other methods. Inclusion
of our method with all other techniques available brings down the current
2{\sigma} contours on the dark energy equation of state parameter wx by about
30%.
