The synthesis of water equation is written on the image i attached. I used stoichiometry and the synthesis definition. Synthesis is a reaction that starts with 2 elements and ends with only one, because the elements were combined. I wrote O + H = H2O. However, O and H are diatomic elements, which means that when they are written by themselves, there cannot be only one of them. So, both O and H have to have a subscript of 2. After that, I balanced the equation so there are 4 of each elements on either side of the equation. I hope this helps you! :))