How do you calculate the equilibrium constant with the absorbance of a substance and the absorbance constant?
I'm following the outline from the comment by user21398.

Initial concentrations after mixing

The volume of the mixture is $V_\text{mix} = \pu{10 mL}$. Using the dilution law, I get the following concentrations:

$$\ce{[Fe^3+]_\text{initial}} = \pu{2.00 mM} \cdot  \frac{\pu{5 mL}}{\pu{10 mL}} = \pu{1.00 mM}$$

$$\ce{[SCN-]_\text{initial}} = \pu{2.00 mM} \cdot  \frac{\pu{2 mL}}{\pu{10 mL}} = \pu{0.400 mM}$$

Equilibrium concentrations

The equilibrium concentration of the complex is already calculated, $\ce{[FeSCN^2+]_\text{equil}}=\pu{6.39e−5 M}.$

To get the equilibrium concentrations of the reactants, we have to consider that some reacted:

$$\ce{[Fe^3+]_\text{equil}} = \ce{[Fe^3+]_\text{initial}} - \ce{[FeSCN^2+]_\text{equil}} $$ $$ = \pu{1.00e-3 M} - \pu{6.39e−5 M} = \pu{0.94e-3 M}$$

And similar for thiocyanide:

$$\ce{[SCN-]_\text{equil}} =  \ce{[SCN-]_\text{initial}} - \ce{[FeSCN^2+]_\text{equil}} $$ $$ =\pu{0.400e-3 M} - \pu{6.39e−5 M} = \pu{0.336e-3 M}$$

Equilibrium constant

This is copied from the question, using the values calculated in this answer above and dropping the unit M because the standard state is 1 M:

$$\mathrm{K} = \frac{[\ce{FeSCN^2+}]}{[\ce{Fe^3+}][\ce{SCN^-}]}$$

$$\mathrm{K} = \frac{\pu{6.39e-5}}{\pu{} \pu{0.94e-3}\cdot \pu{0.336e-3}}$$

$$\mathrm{K} = 202$$