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

The variables should add up to equal 1