Take a look at the item again:
One day in class, Shantrell rested her head against the
wall (A)
and discovered (B)
that she could hear the classical music that Mr. Nguyen played in
his office on the other side. (C)
The music promptly put her to sleep.
You wanted to do this:
One day in class, Shantrell rested her head against the
wall. And discovered that she could hear
the classical music that Mr. Nguyen played in his office on the other side.
The music promptly put her to sleep.
A period after wall makes a fragment. And discovered that she could hear ... is a lonely verb phrase, so it cannot stand alone as a complete sentence.
You might want to review the rules.
Go back to the sentence to try again.