p^2 + 2pq + q^2 = 1

where:

p is the frequency of the dominant allele

q is the frequency of the recessive allele

We are given that 28 organisms are p.

This means that the frequency of the p allele is:

p = 28 / 56 = 0.5

We can then use the Hardy-Weinberg equation to solve for q:

q^2 = 1 - p^2

q^2 = 1 - 0.5^2

q^2 = 0.75

q = 0.866

Therefore, the frequency of the recessive allele is 0.866.