Suppose that the father is homozygous dominant for polydactylism and homozygous recessive for normal digit length. The mother is heterozygous for polydectylism and homozygous dominant for brachydectylism

The way to deal with a dihybrid cross is to draw it out in a punette square. Give each gene a letter say T for tall. TT would be homozygous dominant for tallness tt homozygous recessive for shortness. Right out the parents genotypes i.e. TT Ll x Tt LL.
Now right one parent down the square and the other across- just extend a small punnett square and fill in the children's genotypes.

This site explains it in pictures.

Give this a go and if you still don't get it post back.

EDIT I hope those aren't people's phenotypes! Eep!