Each gamete from the zebra and the donkey is haploid. That is, it has half the number of chromosomes as the diploid numbers you're given in the problem. So you divide each of those by two. The zebra's haploid number is 23 and the donkey's haploid number is 31.
The zonkey gets one haploid gamete from the zebra and the other haploid gamete from the donkey. So the number of chromosomes it has is the sum 23 + 31 = 54.
That should be enough to get you started...
if you still need help ask
