In radishes, red and white are pure-breeding colors, while the hybrids are purple. if av red radish is crossed with a white raddish, what will be the phenotype of the f2 generation

Pure breed essentially refers to the genotypes being homozygous (both alleles are the same, for example RR or rr.) If red represents RR and white rr, assuming one has complete dominance over the other, crossing RR x rr would give us:
Rr no matter how times your cross the parents! Why? Because the red radish can only give one type of allele R, and the white can only give r to its progeny, thus all the radishes in your f1 (first generation progeny) will always be Rr, which is a hybrid that looks purple.
The f2 essentially means you've crossed your f1 generation together, and we already said that they will ALL be Rr; so we know you're going to cross Rr x Rr.
 Rr x Rr yields a ratio of :
1 RR: 2 Rr: 1 rr   (25%, 50%, 25%). 
 Now convert these genotypes to phenotypes and you've got your answer.

The phenotype of the offspring will be 100%tall