You drive 6.0 km at 50 km/h and then another 6.0 km at 90 km/h. your average speed over the 12 km drive will be

V1=50km/h

v2=90km/h

s1=s2=6km

s=12km

The average speed over the 12 km drive:

v=s/t

The time needed to drive 12km will be:

t = t1+t2

t1=s1/v1, t2=s2/v2

t=s1/v1+s2/v2 = (s1v2+s2v1) / (v1v2)

v=sv1v2 / (s1v2+s2v1) = 12*50*90 / (6*50+6*90) = 64,28 km/h