For the Tall Pink Plant we'll call Parent 1.
For the Short Pink Plant, we'll call Parent 2.
Parent 1 has the Genotype: TtRr, because it's Tall Heterozygous (They told you that), and it's pink which would mean it's Rr.
Parent 2 has the Genotype: ttRr, because it's Short (Which has to be homozygous since's its recessive to tall) and it's pink meaning it's Rr.
You're fusing gametes to form a child plant.
For Parent 1, what possible gametes could you form with the genotype TtRr?
You can have Gametes that are TR, Tr, tT, tr.
For Parent 2, what possible gametes could you form with the genotype ttRr?
You can have Gametes that are tR or tr (since Parent 2 only has the t genotype, it can't form a gamete that's T)
Now when you cross these gametes, you can form the Punnett Square at the top.
All plants that are Rr will be pink. All plants that have 1 T will be tall.
You can't ever get homozygous (tall) TT children because Parent 2 is tt homozygous. Each parent has to donate 1 T/t and 1 R/r, and only Parent 1 has a T.
And they're not asking for Short and Pink, they're asking for what's the probability of Tall and Pink. So looking for ttRr is not correct.
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Alternatively, you can solve for probability of Tall from the cross of Parent 1 and Parent 2, and the probability of pink from Parent 1 and Parent 2 and multiply them, provided there's no linkage and the events are independent probability wise.
So Probability of getting a Tall from Parent 1 (Tt) x Parent 2 (tt) would be 1/2.
Probability of getting Pink from Parent 1 (Rr) x Parent 2 (Rr) would be 1/2.
(1/2) x (1/2) = 1/4
