We must factor the denominators first. And to factor a quadratic equation, you need its roots.
We will use the fact that any quadratic equation with leading coefficient 1, i.e. in the form
has the following properties:
- The sum of the roots is
- The product of the roots is
So, for the first denominator, we're looking at two numbers such that their sum is -4 and their product is -12. It's easy to find out that those numbers are 2 and -6.
Similarly, for the second denominator, we're looking at two numbers such that their sum is -7 and their product is 6. It's easy to find out that those numbers are -1 and -6.
So, you can rewrite the expression as follows:
The least common denominator is formed by the factors of the two denominators, and you have to select repeating ones only once.