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 +1 = (x - (-1))`:

`-1`	`1`	`-8`	`-21`	`7`
		`-1`	`9`	`12`
	`1`	`-9`	`-12`	`19`

The last entry of the row is the remainder, `19`.

So the value of `x^3-8 x^2-21 x+7` when `x = -1` is `19`.