Both plants are heterozygous for purple flowers. use the letter p.

On a punnet square, let P equal the variable. 
    P    p 
P PP  pp 
p Pp   pp

25% of the offspring will be Homozygous dominant, 50% will be Homozygous recessive, and 25% will be Heterozygous dominant. 

Note: 
Heterozygous means Pp since "Hetero" means different.
Homozygous means PP or pp since "Homo" means same. 

Idon't know how to do this but maybe ask your mom or dad