I'll assume 
is completely dominant over 
. Recall the Hardy-Weinberg equations:
where
represents the allele frequency for brown fur, or the number of copies of the allele within the population; and 
represents the allele frequency for white fur, or the number of copies of . In the first equation, the squared terms refer to the frequencies of the corresponding homozygous individuals, while 
is the frequency of the heterozygotes.
We're told that 12 individual moles have white fur, so we know for sure that there are 12 homozygous recessive individuals, which means
from which it follows that
Over time, H-W equilibrium guarantees that the allele frequencies
do not change within the population. For example, suppose we denote the frequency of the allele in generation 
by 
. Then
That is, the frequency in the -th generation has to match the frequency of attributed to the 
and half the 
individuals of the previous generation.
The same goes for
.
where
We're told that 12 individual moles have white fur, so we know for sure that there are 12 homozygous recessive individuals, which means
from which it follows that
Over time, H-W equilibrium guarantees that the allele frequencies
That is, the frequency
The same goes for