Did the answer surprise you? Were you expecting an answer of 27 instead of 17?
Mathematica does multiplications and divisions before it does additions and
subtractions, an idea that should be familiar to you from algebra. Using
another idea from algebra, how do you suppose you can force Mathematica to do the
addition first?

Click here for the answer

Simply use parentheses to group the expression differently, like this:

(5+4)*3