When a question states that the plants are "homozygous for the contrasting traits," that means that the plants will have the same letter genotype for both of their chromosomes. In other words, if we use the letter "b" as our chromosome, one plant will have the genotype "BB", and the other will have "bb." Now, there will only be on expected genotype with these, and that would be "Bb." To find this, you must put the genotypes in a punnet square and calculate the different genotypes. The big B is dominant over the little b, so in this case the offspring plant will have a heterozygous for both traits.