This is a way to factoring trinomials (there exist different equivalent methods).
Multiply the trinomial but the term accompanying . This is the second line. Then, you could take the square of the
, ant try to create a factor () () that will correspond to the expression in the second line. That is, we want
In ? we put the corresponding numbers that, if we multiply them we will obtain 42, and if we add them we will obtain 13. This numbers are 6 and 7. Then, we have
The last step is divide by the number that we multipy in the first step.