for the "common denominator" is perfectly fine to use the product of all denominators, the "Least" part simply means the smallest we can use, since the product of all can be simplified further and yet be still divisible by all denominators, but for the sake of an operation the product of all works fine, the only thing is that we'd have to do simplification after, which is not biggie.
Now, there are many times when the LCD is just that, the product of all, since it can't be simplified further, this is one of those cases, so the LCD will just be x(x-12).