A tree diagram is developed by having lines for each choice a person can make at a every branch of the tree. For instance, if you have 3 shirts, and 2 shoes, a tree diagram might be:

Shoe 1 "Shirt 1 and Shoe 1"

Shirt 1 -

Shoe 2 "Shirt 1 and Shoe 2"

Shoe 1 "Shirt 2 and Shoe 1"

Shirt 2 -

Shoe 2 "Shirt 2 and Shoe 2"

Shoe 1 "Shirt 3 and Shoe 1"

Shirt 3 -

Shoe 2 "Shirt 3 and Shoe 2"

See if you can form a conjecture about the number of items in the shirt column, the number of items in the shoe column and the total combinations in your final column.