Let rate of faster bus be x km/h and rate of slower bus be y km /hr.
The relation between rate of slower and faster bus is,

Two bus are travelling in opposite direction so relative speed is,

Two buses meet in 6 hours so,

Substitute y + 12 for x in the equation to obtain the value of y.

Determine the value of x.

So answer is,
Rate of faster bus is 123 km/hr
Rate of slower bus is 111 km/hr.