Try asking in the questions area of the lesson where you learned this. From what I understand, here’s the answer:

Since the mother has type O blood, she must have two O alleles as O is a recessive blood type. The kids have genotypes AO and BO resulting in A and B blood types. Since the mom had neither A nor B alleles, they must have both come from the father. This means that the father had type AB blood. It’s really just like algebra.