This note may appear to be long and complicated. But you can easily understand it if you put in your hands a pencil and paper and have ready for reference the two references I state below. Write the variables and terms in your own symbols as you read.
Error analysis is explained in ref.
Error analysis is explained in detail in ref. 2, pages 13-20 and for error in slope. There are four spectra. Recall the different extinction coefficients in table 1 for the two compounds in the two different solutions. From your Absorbance values, calculate the concentrations of these solutions.