Their child can not have a widow's peak, seeing as widow's peak is a dominant trait. They need two recessive alleles to have a straight hairline, meaning none can be inherited by the child.
Hope this helps.
The solution is 0%.
In order to the parents not to have a widow's peak, they would need homozygous recessive. This is the only allelic combination where they lacked one dominant form of allele. All children will not have widow's peak because neither parent has the possible allele.
