One trait a plant can have is either red leaves (r) or brown leaves (r). another trait is tall stems (t) or short stems (t). a plant used in a dihybrid cross has the genotype rrtt. what is the phenotype of this plant?

The upper-case letters generally refer to a dominant allele, meaning that if it is present in the genotype, it will be present in the phenotype. Therefore, a plant with genotype RrTt has the dominant alleles for both traits, and thus expresses these genes, giving a phenotype of a (R)ed leafed plant with (T)all stems.