If you bet on a single number (i.e. 00, 0, 1-36), there is a one in 38 chance of winning. The payout is only 35x, so if you bet on every slot you would have to pay $38, and you would be guaranteed to get $35. So the house makes a guaranteed margin of $3. The house’s margin is $3/$38, i.e. 7.9%.