i think you need to look at a calibration curve…

You can’t really answer the question unless you’ve been given the wavelength at which the absorbance was measured–or use a calibration curve like MrMistrey said, and even then they probably should at least tell you the wavelength. You need to use Beer’s Law and the molar absorbtivity (molar extinction they called it in the Elder Days) at the wavelength used to calculate the concentration of NADH in the cuvet. The coefficient is wavelength-dependent; the value changes dramatically sometimes over very small ranges of wavelengths. Then you know you made a 1:3 dilution of NADH:water so you can multiply the concentration of NADH in the cuvet by 4 to find the concentration in the original solution.