Take a look at the original sentence again:
In Dr. Ribley's calculus class, Yi-Ping tapes every lecture and carefully
rewrites her notes, she is planning to sell
these guides to desperate freshmen next semester.
You wanted to fix it this way:
In Dr. Ribley's calculus class, Yi-Ping tapes every lecture and carefully
rewrites her notes, because she is planning
to sell these guides to desperate freshmen next semester.
When you have two main clauses incorrectly joined together, one way to fix the problem is to subordinate one of them. Because is a subordinate conjunction that will do this job. You must remember, however, that when because follows a main clause, you should not use a comma with it. In fact, 99.99% of the time, you do not use a comma before because.
You might want to consult the rules for fixing comma splices and fused sentences.
Go back to the sentence to try again.