trial and error
either
(z )(21z )
(3z )(7z )
and ending with
( -1)( 26)
( -26)( 1)
( -2)( 13)
( 2)( -13)
trial enderor
after some times
(3x+13)(7x-2)=0
set to zero
3x+13=0
3x=-13
x=-13/3
7x-2=0
7x=2
x=2/7
x=-13/3 or 2/7
it would be easier to use quadratic formula though