94. Solving a chemistry problem

In the spectrophotometric determination of iron (III) by reaction with sulfosalicylic acid (pH 8.2) in volumetric flasks with a capacity of 100 ml, five standard solutions were prepared containing an iron (III) complex with sulfosalicylic acid, with different precisely known concentration c (Fe) of iron ( III). The optical density A of these solutions was measured at a wavelength of 420 nm in a cuvette with an absorbing layer thickness of 1.00 cm relative to water (reference solution) and obtained the following results:
C(Fe), μg/ml 1.00 2.00 4.00 6.00 8.00
A 0.098 0.197 0.395 0.590 0.785

An aliquot of the analyzed solution with a volume of Va was taken into a volumetric flask with a capacity of 100 ml, and after carrying out a photometric reaction and diluting with water to the mark, the optical density A(X) of the resulting solution was measured under the same conditions under which the optical density of the reference solutions was measured.
Build a calibration graph in the coordinates A - c(Fe), µg/ml. After making sure that the basic law of light absorption is satisfied, at the above wavelength, calculate the average specific extinction coefficient E of the complex of iron (III) with sulfosalicylic acid in solution in terms of the iron cation Fe3.
Based on the found value, taking into account the data below, determine the concentration (mcg/ml) of iron (III) in the analyzed solution.
Va, ml 5.00
A(X) 0.346