The remainder theorem tells us the value of a polynomial when `x = a` is the remainder when the polynomial is divided by `x - a`.

We'll use synthetic division to divide by `(x - 5)`:

`5`	`1`	`-3`	`-9`	`3`
		`5`	`10`	`5`
	`1`	`2`	`1`	`8`

The last entry of the row is the remainder, `8`.

So the value of `x^3-3 x^2-9 x+3` when `x = 5` is `8`.