The Beer's Law plot is a graph of Absorbance data vs Concentration data. The plot is linear which follows the slope-intercept form: y = mx + b, where m is the slope and b is the y-intercept. The y data is the absorbance, and the x data is for the concentration. At the same wavelength, this equation can be used to determine C at a given absorbance. When y = 0.31, then x is
0.31 = 3.2975x + 0.0021
Solving for x,
x = 0.0934
Thus, the unknown concentration is 0.0934 mol/L.