Take a look at the sentence again:
Davina, a good friend from my biology class, has
more interest in what is under a guy's skin than what is under
his clothes, for this reason, she is pursuing a degree in radiography.
The comma between class and has is correct. Davina has more interest ... is a main clause. A good friend from my biology class is an appositive, a noun phrase. In the example above, the appositive is interrupting the flow of the sentence, so it requires a comma both in front and behind it.
Go back to the sentence to try again.