We use quadratic formula here (for these questions, it is difficult to do cross factorisation). This formula works for all quadratic equations, regardless of whether the quadratic expressions are factorisable by cross factorisation or not.

As a side note, the discriminant b2 - 4ac that you have been seeing for the past few days in A Maths is a part of the quadratic formula. This is an important study in maths because the quadratic formula contains the square root of this discriminant b2 - 4ac. If the discriminant is negative, then you will be facing a square root of a negative answer, which explains why b2 - 4ac < 0 leads to 'no real roots'.