As they are similar figures,we can use the ratio of the side to find out the value of z.
The only side that both have the value indicated is 4 and 6,therefore we would use it to find the ratio.
The eqaution would be:
6/4 = z/8
3/2 = z/8
3×8 = 2z
z = 24/2
z = 12
Thus the value of z is 12.
Hope it helps!