200 cc of x × 10^ -3 M potassium dichromate is required to oxidise...

Mohr's salt ($FeSO_4 \cdot (NH_4)_2SO_4 \cdot 6H_2O$) is a source of $Fe^{2+}$ ions.
The redox reaction with potassium dichromate in acidic medium is:
$6Fe^{2+} + Cr_2O_7^{2-} + 14H^+ \to 6Fe^{3+} + 2Cr^{3+} + 7H_2O$.
Moles of $Fe^{2+}$ from Mohr's salt solution:
$n(Fe^{2+}) = 0.6 \text{ M} \times 0.750 \text{ L} = 0.45 \text{ mol}$.
From the stoichiometry, 6 moles of $Fe^{2+}$ require 1 mole of $Cr_2O_7^{2-}$.
Moles of $Cr_2O_7^{2-}$ required:
$n(Cr_2O_7^{2-}) = \frac{0.45}{6} = 0.075 \text{ mol}$.
Molarity of potassium dichromate:
$M = \frac{0.075 \text{ mol}}{0.200 \text{ L}} = 0.375 \text{ M} = 375 \times 10^{-3} \text{ M}$.
Therefore, $x = 375$.