Take another look at the sentence:
As her teachers had recommended, Madison
tried to study in the library but discovered that the good music, unlimited
soda, and big tables at the campus café made a great place to do
homework during fall semester, Madison must have eaten 150 pizza slices
while she slogged through Fundamentals of Biology.
The comma between recommended and
Madison is completely correct.
As her teachers had recommended is a
subordinate clause, so it cannot
stand alone. Madison tried to study in the library
... begins a main clause. When
a subordinate clause introduces a main clause, you need a
comma between them.
Go back to the sentence to try again.