If I have a graph of concentration in Absorbance at 400nm vs [micromolar]of para-nitrophenol

Question

The slope would be the extinction coefficient. (y=mx) if forced through 0
The problem is what is y?
I know y is the absorbance but it's given on a paper i have as, y= Absorbance at lmax

Can someone explain what lmax means?
I need to find the concentration of an unknown from this standard curve and I think I find it by first finding the epsilon from the graph, by using..

1. y=mx then:
m= # microM^-1 cm^-1
(not sure if it needs to be in M^-1 or how to convert it to that if I need it to be)
The m is the epsilon.
The problem is that the graph's slope is 0.003653 is molar absorption coefficient supposed to be that small?

2. then I take that and use it to find the concentration of the unknown solution, from beer's law, since I have the absorption of it.
Another issue is that the sample I did measure, was of a solution that I diluted in a ration of 1ml sample to 3ml pH10 buffer. The standard curve sample of 0.1M Paranitrophenol was also diluted with buffer prior to being analyzed, with same ratio of dilution.
First, finding the concentration of the diluted unknown is okay, as long as the absorption coefficient is right...by using A= ebc
A= known
e= calculated from graph above
c= concentration (can be found)
b= 1cm

But how am I to find the original concentration of the unknown stock paranitrophenol if I did not analyze it and was not told to....instruction ssay to, "calculate the concentration of the diluted unknown Paranitrophenol and the original unknown stock paranitrophenol using the standard curve generated"