Multiply :

x + 4 by x - 5

(x + 4) $\times$ (x - 5)

= x $\times$ (x - 5) + 4 $\times$ (x - 5)

= x $\times$ x - x $\times$ 5 + 4 $\times$ x - 4 $\times$ 5

= x² - 5x + 4x - 20

= x² - x - 20

Hence, (x + 4) x (x - 5) = x² - x - 20