Dad: TtBb
Mom: ttbb
You have to use distribution for dihybrid crosses. Meaning, the first allele of each trait has a equal chance of being paired with the other allele of the other trait. So for example with Dad, I will number the traits:
T(1)t(2)B(3)b(4)
To set up the possibilities from Dad, it would be 13, 14, 23, 24: TB, Tb, tB, tb. Same idea goes for Mom, except since all alleles are the same, you only need to make one column for Mom, since if you did all 4, the other 3 would just be repeats of the 1.
Cross:
tb
TB TtBb
Tb Ttbb
tB ttBb
tb ttbb
The phenotypic ratio is 1 Tall Brown: 1 Tall Blue: 1 Short Brown: 1 Short Blue