First let's discover which element is oxidizing and which is reducing
So first let's rewrite the equation here:
Cr2O7^2- + C2O4^-2----> Cr^3+ + CO2
Reactant side:
oxidation state for Cr:
2x + (-2x7) = -2
x = +6
oxidation state for C:
2x + [4x(-2)] = -2
x = +3
Product side:
Cr - +3
oxidation state for C:
x + [2x(-2)] = 0
x = +4