Answer:
2Ca+2O2->2CaO
Step-by-step explanation:
You have 2 O’s on the left side so you multiply the right side by 2.
Ca + O2 -> 2CaO
Now that you have 2 Ca on the right side and only 1 on the left, you multiply the Ca on the right by 2.
2 Ca + O2 -> 2 CaO