oxygen, which is what that letter "O" stands for.
KClO3 -----> KCl + O2 first you stability the equation: 2KClO3 -----> 2KCl + 3O2 ratio KClO3 to KCl is two:2, it quite is a million:a million so which you like 27 moles of KClO3 to offer 27 moles of KCl
just look at the formula
what is repeated
sometimes it's hard to see the forest for the trees
all the best