Is the expression 3x+2x−4 in simplest form

No, because you would have to simplify like terms. Which the simplest form would be
5x-4

Her conclusion that 90% of the student body wants the colors changed is an INCORRECT conclusion. The 90% out of 100% is not referring to the total numbers of the student body.For example, 100 people are members of student body if only 50 members responded the survey, then we have:
 90% of 50 members who answered the survey said that they want the changed.