Make the equation equal on each side. with number 1 it says the left side of the arrow has 1 sn, 2 s, and 2 o. the right has 1 sn, 4 o, and 1 s.so you have to balance the equation by putting numbers in front of the compounds.sns2 + o2 = sno2 + so2sns2 + 3 o2 = sno2 + 2 so2now there is 1 sn, 2 s. and 6 o on each side. the large number in front of the compounds essentially multiplies it by that the subscript of that number. so with o2 it says there are 2 oxygens. so they come in pairs. if you get 3 pairs of oxygens there will be 6.the same process goes with the rest of them too. sometimes it takes a few guesses. just figure them out like an algebra problem. put the equal sign between them like i did. it maked it easier for me.