The first 3 common multiples of 5 and 15 are 15, 30, and 45. Remember that the number itself is also a multiple of any number, since you can multiply it by 1.
The quotient is -8. This being because when a negative number is in parentheses, the negative will be the opposite: Positive. And we know 48 divided by 6 is 8. So the answer is B.