I need help with how to do Hardy Weinberg problems?

The letters p and q each represent the frequency of a given allele of a gene.  If we're looking at a gene for plant height, for example, we can say that p represents the frequency of the 'tall' (T) allele and q represents the frequency of the 'short' (t) allele.  If these are the only two alleles for the gene, then when we add their frequency together, it should equal one.  So we get the equation

p+q = 1

To find the frequency of homozygotes and heterozygotes, we square both sides of the equation to get

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

Here, p^2 represents the frequency of plants homozygous for the tall allele (TT), 2pq represents the frequency of heterozygotes (Tt), and q^2 represents the frequency of plants homozygous for the short allele (tt).

Suppose the frequency of homozygous recessive (tt) is 0.16.  This means that q^2=0.16, so q=0.4.  This can be used with the first equation (p+q=1) to calculate that p=0.6. Thus the frequency of T is 0.6 and the frequency of t is 0.4.  You can use the two p/q equations given together to solve HW problems.