Explanation:
According to the Beer-lambert' s law,
         Absrobance = absrobptitvity × lenght × concentration
So, if we will plot a graph between absrobance and concentration then we will obtain a straight line
Hence, formula to calculate slope of the graph is as follows.
    The slope of graph =  
The given data is as follows.
       = 0.412 (first point)
 = 0.412 (first point)
       = 0.642 (second point)
 = 0.642 (second point)
       = 0.240 ppm (first point)
 = 0.240 ppm (first point)
       = 0.475 ppm (second point)
 = 0.475 ppm (second point)
Hence, putting these values into the above formula we will calculate the value of slope as follows.
            Slope =  
                      = 
                       = 
                      = 0.978 
It is known that for the graph, line equation can be written as follows.
                        y = mx + c  ........... (1)
where,      C = intercept
                  m = slope
Hence, calculate the value of intercept as follows.
            0.412 =  + c
 + c
                c = 0.178
As it is given that the absorbance values 0.454 (the y-axis value). Therefore, putting this value into equation (1) we get the following.
                      y = mx + c
                   0.454 =  + 0.178
 + 0.178
                    0.276 = 0.978x
                          x = 0.282 ppm
Thus, we can conclude that the concentration of Pb in the given sample is 0.282 ppm.