In some plants, height is polygenic character. This means it is controlled by multiple genes. It is the presence of dominant alleles that decide the increase in height. More the dominant alleles, more will be the height of a plant.
In the question, it is given that height is controlled by three pairs of genes (6 alleles). According to the values, the maximum height given is 18 feet for AABBCC and minimum height is 6 feet for aabbcc.
Therefore, each dominant allele contributes = 2 feet
The plant with genotype AabbCc has 2 dominant allele and hence the height of the plant would be 6+2+2=10 feet.