Good afternoon.
In the reference of the second car, itself is stopped! It is the same thing as adding the opposite speed vector of this car to the rest of the world. See below:
Begginning:
Car 1 Car 2
→ 85 km/h → 54 km
Going to the referential of car 2, adding ← 54 km/h to both cars:
New Reference:
Car 1 Car 2
(→85 km/h) + (←54 km/h) (→ 54 km/h) + (← 54 km/h)
Car 1 Car 2
→ 31 km/h 0 km/h
So, relative to car 2, car 1 has a velocity of 31 km/h