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)`:

`1`	`1`	`7`	`9`	`-4`
		`1`	`8`	`17`
	`1`	`8`	`17`	`13`

The last entry of the row is the remainder, `13`.

So the value of `x^3+7 x^2+9 x-4` when `x = 1` is `13`.