Here it says write a formula for sodium peroxide. Well, we know sodium is in Group 1A, so it's plus one in charge. But peroxide? What does peroxide look like? Well, OPS oxides, peroxides, superoxides. And we know that it has a number of 1221 peroxide which is P.
Here, remember the first number tells us the number of oxygens which is 2 and the second number tells us the charge 2 minus. So this is the formula for peroxide. The numbers in the charges are different so they don't cancel out the crisscross. One comes here, 2 comes here. So sodium peroxide would be Na2O2.
This would be our final answer.