Where are you getting these lines?
In general 3 point favorite wins 60%, 6 point favorite wins 70%, 9 point favorite wins almost 80% etc....
The Jets have a 40% chance of losing, the Broncos have a 20% chance of losing. For the Bills this is an either or so we add those = 60%
The Bills favored by 9 have an 80% chance of winning outright.
since we need a Bills win and a Jets or Denver loss 80% X 60% = 48% chance of playoff birth