In fruit flies, the gene for red eyes (r) is dominant and the gene for sepia eyes (r) is recessive. if two red-eyed heterozygous flies (rr) mate together, what are the possible genotypes of their offspring? (hint: you many want to complete a punnett square.)

You have 2 parents both Rr, so the cross will simply be Rr x Rr. Drawing a punnet square, your cross should look like this:
We see that when 2 heterozygotes cross, the possible genotypes for their offspring are RR, Rr, and rr. The punnet square tells us that 25% of the offspring will be RR, 50% will be Rr, and 25% will be rr. Phenotypically, this means 75% of offspring will have red eyes, and 25% will have sepia.