Aplant can produce either purple flowers or white flowers. what is the probability of purple-flowered offspring if two plants that are heterozygous for purple flowers are crossed?

You would be crossing Pp and Pp. If you put them in a punnet square, the four possible gene combination you get would be PP, Pp, Pp, and pp. In all cases except the last, the resulting flower's phenotype would be purple. So the probably of purple flowered offspring is 75%, or 3/4.