You need to draw a punnet square:
  _______
__|P  | w  |
|P |PP| Pw|
|w |Pw| ww|
So that means you'll have:
1/4 PP flowers, which are homozygous purple flowers
2/4 Pw flowers, which are heterozygous purple flowers
1/4 ww flowers, which are homozygous white flowers
So the ratio of purple to white flowers is 3:1Â