Do the calculation for each column and average the concentrations of K dichromate. Note that the middle column was an overshoot and you could make a case for omitting it in the average.

You need the stoichiometry for the redox reaction:

Fe^{2+} --> Fe^{3+} + e^{-}

Cr_{2}O_{7}^{2-} + 6 e^{-} +14H^{+} --> 2Cr^{3+} + 7H_{2}O

You can work out the overall reaction. What matters is that 1 dichromate ion requires 6 Fe^{2+}

If you use stoichiometry to find molarity, you should find the moles and then divide by the amount of fluid you started with.

Column 1 :

(12.6 ml - .4 ml) (1 liter/1000 ml)(.0151 moles FeCl_{2}/liter of soln)(1Fe^{2+}/FeCl_{2})(1 Dichromate/6 Fe^{2+})(1 Pot. Dichromate/1 Dichromate) = moles of potassium dichromate

To find the molarity divide the moles by the starting volume(in liters) of the dichromate soln which is not given.

Please consider a tutor.