How do I tell my professor he's wrong?

So, I studied this course in detail before the semester started. While he was solving a question, he made an error, though a small one(he mixed up a formula and it affected the whole solution) I raised my hand and tried to point it out to him but he didn't get what I was saying. Then, some guys were telling me to 'shut up and do it 'his' way' whenever a test or exam comes but I was very sure it was a mistake and I want to correct it without sounding rude because its also a very slight mistake