How do I calculate the frequency of Phenylketonurioa caused by zugosity for a recessive allele?

Phenylketonuria is caused by zygosity for a recessive allele and can cause a severe form of mental retardation. this condition affects 1 in 10,000 newborn Caucasian children. calculate the frequency of heterozygotes for this allele( ume the genotypes are in Hardy-Weinberg equilibrium).