stubborn donkey is right both are perfectly valid
however I contacted the math department and was explained this (it was my old trig prof too
"unless the work clearly states not to, use pemdas when reading or expressing a math problem"-Dr.Jow (Mathematician and Nasa engineer)
^I don't think anyone wants to argue with that the PhD gets the final word
I had one royal bitch of a time explaining that to my ex through her math courses I'm like
"I don't care what you fancy AP teacher told you about it!!! Your college professors and classmates will apply it when reading your work"
" Though I be shamed, not once did I forget my knightly vows..if I could protect but one person from war's horrors, I would bear any shame...I would bear it proud!"
-Basch Von Ronsenburg of Dalmasca,
Final Fantasy XII