Answer:
v = 1.36 cm / y
Explanation:
For this exercise we must assume that the displacement of the plates is constant over time, so we will use the kinematic relationships for the uniform movement
   v = d / t
We reduce the quantities to the SI system
   d = 320 km (1000 m / 1km) (100 cm / 1 m)
   d = 3.2 107 cm
let's calculate
   v = 32.107 / 23.5 106
   v = 1.36 cm / y