Question

How do you multiply `x(x-5)(x-4)`?

Answer 1

`x(x-5)(x-4)`

Multiply two terms at a time
`(x) xx [(x-5)(x-4)]`

`= (x) xx (x^2-9x+20)`

`=x^3-9x^2+20x`

Stop here if you understood the above

Since you are still reading the most likely problem is in multiplying binomials.
There are numerous ways to approach this; perhaps the following will help:

`( (xx,,x,+,(-4)), (,-,-,-,-), (x,|,color(blue)(x^2),,color(blue)(-4x)), (+,|,,,), (-5,|,color(blue)(-5x),,color(blue)(+20)), (,-,-,-,-), (,,color(red)(x^2),color(red)(-9x),color(red)(+20)) )`

or maybe
`(x-5)xx(x-4)`
`= (x)(x-4) - (5)(x-4)`
`=(x^2-4x) - (5x-20)`
`=(x^2-4x-5x+20`
`=x^2-9x+20`