Given Z is directly proportional to X and inversely proportional to Y.
This means that
Z=kX/Y where k is a yet-unknown constant.
We are also given that, when X=5, y=19, Z=2.8947368421053, this means
2.8947368421053=k(5/19)
or
k=2.8947368421053*19/5
=11.0000000000001
=>
Z=11.000000000000*X/Y
If X=12, Y=24, then
Z=11.000000000000*X/Y
=11.000000000000*12/24
=5.500000000000