The first thing Alan must choose is the type of greenery. There are two choices, so your tree diagram should split into two branches.
For each of those two choices, he must choose between three different types of flower. Draw each branch splitting into three different branches.
There are six different possibilities.
How many of the six possibilities contain ferns?
Count them using the tree diagram.
Look at the diagram and see how many of the possibilities do not include sunflowers.