Fix the two words that are used incorrectly. On
their
road
trip
from
Mexico
two
the
Grand
Canyon
Christine
and
Nora
past
through
the
cities
of
Tucson
Phoenix
and
Flagstaff

The first mistake is the word 'Two' and the second mistake is 'Past'.

Explanation:

Two, is meant to be spelled To

Past, is meant to be spelled Passed

quite an interesting way to teach topic sentences, because the answer to this question is the best definition of a topic sentence that one can present, it is as if answering the questions taught you how to do it, this is some real teaching, it is terribly well-done.

the topic sentence in this paragraph is:

topic sentences writers stay on track and keep a paragraph from becoming an aimless collection of sentences about an idea.

which incidentally tells you what a topic sentence should be.