A woman with a straight hairline has as child with a man who is heterozygous and has a widow's peak.
What is the mother's genotype?

aa

Explanation:

A widow's peak is the dominant genotype so one must not have that trait. If it was presented then the woman would have the genotype.

A widow's peak is dominant so the man would be either AA or Aa. Since it stated that his is heterozygous (Hetero meaning two different), his genotype would be Aa (again since a widow's peak is the dominant, it only needs one dominant genotype)