then Z 4 comma X for given X 3 it's what we have over here then we sum over X 4 so we sum out X 4 so we end up with here is P Z 4 X 4 is being summed out given X 3 all right now as we keep processing this and this will in the future call a backward message be X 3 X 3 is the variable then once we multiply in Z 3 given X 3 what do we what do we get we get I should we multiply multiplying so multiplying this one Z 3 given X 3 which would give us pc3 comma Z forgiven X 3 if we go up to here then we'll apply X 3 given X 2 which will make it multiply