Equivalent mass of  $Fe{C}_2{O}_4$ in the change,  $Fe{C}_2{O}_4\to F{e}^{+3}+C{O}_{2}isM/x,xis$

$F{e}^{+2}\to F{e}^{+3}+e^{-}$

$C_2{O}_4^{-2}\to 2C{O}_2+2e^{-}$

Adding both the reactions , we get

$F{e}^{+2}+C_2{O}_4^{-2}\to 2C{O}_2+F{e}^{+3}+3e^{-}$

Equivalent weight is defined as the molecular weight divided by the number of moles of electron required by the molecule.

Hence equivalent weight is equal to $\frac{M}{3}=\frac{M}{x}$

Therefore $x=3$.