Take a look at the sentence again:
Latoya tried to concentrate on Dr. Lemon's dull art history lecture, but
the darkened classroom and endless slide show soon had her dozing in her
seat, Latoya knew that Beverly would poke her with a
pencil if Dr. Lemon bothered to glance their
way.
No punctuation is necessary between pencil and if. That Beverly would poke her with a pencil is one subordinate clause. If Dr. Lemon bothered to glance their way is a second subordinate clause. Subordinate clauses are incomplete thoughts and cannot stand alone as sentences. Generally, when you connect two subordinate clauses together, you need no punctuation.
Go back to the sentence to try again.