***See attached pic***
Here's a made-up example to prove the point.
A = Tall plant
a = short plant
B = Big leaves
b = small leaves
(assuming 'A' is completely dominant to 'a', and 'B' is completely dominant to 'b')
If the parents are heterozygous for both traits then their genotypes will be "AaBb".
Make a 16-square Punnet Square and place all of the possible allele combinations for each parent gamete at the top (gametes are the sperm/egg and they have 1 copy of allele A/a and B/b --> if you've taken Algebra and you know how to FOIL something then do that to each genotype to find the possible allele combinations for each gamete).
Once you've created the Punnet Square you will see that:
9 plants = Tall and Big leaves (***see pic -blue squares***)
3 plants = Tall and small leaves (***see pic -purple squares***)
3 plants - short and Big leaves (***see pic -green squares***)
1 plant = short and small leaves (***see pic -orange square***)
RATION = 9:3:3:1
