-5x + 9 + 2x - 4 simplifies into the expression -3x + 5.
First, gather the like terms. Your problem should look like: (-5x + 2x) + 9 - 4.
Second, simplify. Your problem should look like: -3x + 9 - 4.
Third, simplify to get the expression. Your problem should look like: -3x + 5, which is the expression.