In a quadratic equation #ax^2+bx+c=0#, to solve one splits the middle term in two parts so that their sum is #b# and product is #ac#.
Hence, in #x^2+x-6=0# one needs to split #1xx(-6)=-6# in two parts whose sum is #1#. It is apparent that these are #3# and #-2#, hence #x^2+x-6=0# can be written as