(x^3+5x^2−9x−45)/(x−3)(x+5).

х^2(х+5)-9(х+5)
=------------------=
(х-3)(х+5)

(х^2-9)(х+5)
=------------=
(х-3)(х+5)

(х-3)(х+3)(х+5)
=----------------=х+3
(х-3)(х+5)