To factorize `z^2 +15z-54` you need to establish the factors that are relevant between the first and the third terms:
`z^2 = z times z` and the factors of 54 are (1 x 54, 2 x 27, 3 x 18,
6 x 9, ) . Use must establish which of these combinations will render a middle term of +15
The only ones that will work are 3x18. Care must be taken not to use 6 x9 which seem s most obvious but would render +54 as the third term .