Dear elliotyang;
Use twice the Beer-Lambert Law:
Absorbance = Epsilon * Concentration * Path-Length
(You may also search for the Law.)
Good Luck!
ARGOS++
Dear elliotyang;
In the kind the question is given to you it's implied that no condition should be violated.
Absorption measurements are done in a so called Cuvette, and such a thing has a path-length.
But it is also implied, that the measurements are done in the same Cuvette and so it's allowed to set its value to 1.0 cm
In the first equation where you know the concentration and the absorbance you solve the equation for the Epsilon and insert its value in the second equation, which you can solve for the concentration.
For more information on Beer-Lambert law you may visit:
"Corrected Beer's Law (http://www.p-forster.com/english/themes/Spectroscopy/BASICS/Corrected Beer's Law.htm)"
I hope to have been of help to you.
Good Luck!
ARGOS++