If a woman is heterozygous for type a and her husband is heterozygous for type b what is the probability of blood types that their offspring would have

For this you would use a punnet square (ill try my best to show it while typing)
heterozygous means different, so (with blood type) with heterozygous A you will see AO (this will be blood type A)

     A     O
B  AB  BO
O AO  OO

you then have the types in the squares:
AB (type AB blood)
BO (type B blood)
AO (type A blood)
OO (type O blood)