In roses, red flowers and long stems are dominant traits. a rose plant that is homozygous for both red flowers and long stems is crossed with a rose plant that is heterozygous for red flowers and homozygous for short stems. what percentage of the offspring will exhibit red, long-stemmed
flowers?

The answer is 100%.

This is an example of Mendelian dihybrid cross (a cross between two different traits).
If:
R - (dominant) allele for red flowers
r - (recessive) allele for non-red flowers
S - (dominant) allele for long stems
s - (recessive) allele for short stems.

Then:
RRSS - a rose plant that is homozygous for both red flowers (RR) and long stems (SS)
Rrss - a rose plant that is heterozygous for red flowers (Rr) and homozygous for short stems (ss)

The cross will be like:
Parents:        RRSS        x        Rrss

Since it is not easy to represent all of the offspring, the Punnett square can be used, or we can take a look separately on traits:

I trait:
Parents:            RR        x        Rr
Offspring:    RR       RR      Rr       Rr
There are only dominant homozygous (RR) and heterozygous (rr). All of the offspring will have red flowers because the trait is dominant.

II trait:
Parents:            SS        x        ss
Offspring:     Ss       Ss      Ss       Ss
There are only heterozygous (Ss). All of the offspring will have long stems the trait is dominant.

When we match these two traits, all of the offspring will have red long stemmed flowers.