Here is the passage again:
Jaime grabbed the leash with both hands and threw his weight backward. To
keep Roxie, his German shepherd, from attacking Mr. Velasquez, who was walking
down the sidewalk.
You chose to use Punctuation Rule 5 to fix the problem:
Main clause + Ø + infinitive phrase.
The correction looks like this:
Jaime grabbed the leash with both hands and threw his weight
backward to keep Roxie, his German shepherd,
from attacking Mr. Velasquez, who was walking down the sidewalk.
Well done! You deserve that huge sandwich!