a population has two alleles, B and b. the allele frequency of B is 0.73.?

Question

what is the heterozygotes frequency, if the population is in H-w equilibrium?

Answer 1

In Hardy-Weinberg equilibrium the heterozygote frequency =
2(frequency of allele p)(frequency of allele q)

In your case that =
2(freq. B)(freq. b) = 2(.73)(.27) = .39

Answer 2

p + q = 1
q = 1 - 0.73 = 0.27

2 pq = 2 * 0.73 * 0.27 = 0.39