How many atoms of hydrogen are present in the reactants and in the products? 6CO2 + 6H2O → C6H12O6 + 6O2

Because it is a chemical reaction, both sides will need to be balanced, but to show that the number of hydrogen on each side is the same, we should actually calculate how many are on each side

6 (CO2) + 6 (H2O)

on this side the only molecule with hydrogen is H2O, and has 2 hydrogen per molecule, while the reaction uses 6 molecules, giving that side a total of 12 hydrogen atoms.

(C6H12O6) + 6 (CO2)

on this side, the only molecule with hydrogen is C6H12O6, and it has 12 hydrogen per molecule, and the reaction only produces one molecule, giving this side a total of 12 hydrogen atoms

12 = 12

12 atoms