Thank you for the opportunity to help you with your question!
Fe is the limiting reactant.
First you need to balance the equation, so you get:
4Fe + 3O2 → 2Fe2O3
Next, observe that you were given the amounts of each substance in moles. If you were given this in grams or kilograms, note that you would have to convert this to moles. Now look at the coefficients for the Fe and the O2 - they are 4 and 3, respectively.
There are many ways to find the limiting reagent, but I just like to look at the mole ratio: 4 Fe/3 O2 = 1.33333. Now compare the actual value of the moles to the value you just got. So, 6.8 mol Fe/8.9 mol O2 = .7640. As you can see, .7640 is less than 1.333, meaning that Fe is the limiting reagent. If the compared value of the moles were bigger than 1.333, then O2 would be limiting.
Please let me know if you need any clarification. I'm always happy to answer your questions.