A spectrophotometer was calibrated with a 5.2×10^-4 M (in Cu) Cu-neocuproine solution. This calibration was carried out in a 1.00 cm cell, and yielded an absorbance of 0.215 AU. An unknown was then analyzed in this same spectrophotometer, but using a 2.50 cm cell. If the absorbance of the unknown was 0.222 AU, what is the Cu concentration?