If you are multiply 5(x+9), then you are essentially multiplying 5(x) and 5(9).
The parenthesis just mean multiply.
5 times x equals 5x
5 times 9 equals 45
Because you are subtracting x and 9 in the parenthesis, you're expression would look like this:
5x-45
So, 5x-45 = 5(x-9)
The expression is 5x-45