Chemistry help, molar m and magnesium oxide!?

So i did a chemistry experiment and have no idea how to answer this question. If i put magnesium into a crucible and then heated it to create magnesium oxide how do i find the amount of oxygen that reacted with the magnesium? The crucible weighed 39.08g, and with the magnesium weighed 39.21g. After I put it on the Bunsen burner, it weighed 31.29g. Any solutions?