Genetic problem help please?

In garden plants the allele for tall (D) is dominant over the alliele for dwarf (d), and the allele for purple (P) is dominant to the allele for white (p). A tall plant with purple flowers crossed with a dwarf plant and white flowers produces 1/2 tall with purrple flowers and 1/2 tall white flowers. What is the genotype of the parents. Can someone explain to me step by step on how to do this stuff. I understand some of it but then i just get confused