Weighted Basis AVG

i am trying to find the average basis of corn and soybean commodities. i have the basis amont that each contract was purchased at, the total bushels each contract bought and how many different contracts was purchased by a certain person. i am not coming up with the correct avg basis. what is the correct formula. Here is how my columns are laid out. 

AccountNo     Basis     Bushels

1                      -0.35      1000

1                      -0.35       950

1                      -0.40      1000

1                      -0.35       2000

Thank you