7.5 gm of chromium and 7.5 grams oxygen yield
This is a limiting reagent (LR) problem and you know that because amounts are given for BOTH reactants.
4Cr + 3O2 ==> 2Cr2O3
mols Cr = grams/molar mass = ?
mols O2 = grams/molar mass = ?
Using the coefficients in the balanced equation, convert mols Cr to mols Cr2O3.
Do the same for mols O2 to mols Cr2O3.
It is likely that these two values will not agree which means one of them is not right. The correct value in LR problems is ALWAYS the smaller value and the reagent responsible for producing that value is the LR.
To find grams Cr2O3, use the smaller value of mols x molar mass Cr2O3 = g Cr2o3.