when you said that a is the constant ,and b is the path length what do you mean??

would the constant be 10 because for wavelength we increased it by 10 each time??

The constant, a, is determined experimentally. A = abc is the formula you use. A graph can be used to determine a if you don't know it. Or a set of data in which the concn is known can be used if c and A or given.