Factor the expression by grouping. First, the expression needs to be rewritten as $x^{2}+ax+bx+9$. To find $a$ and $b$, set up a system to be solved.
$$a+b=-10$$ $$ab=1\times 9=9$$
Since $ab$ is positive, $a$ and $b$ have the same sign. Since $a+b$ is negative, $a$ and $b$ are both negative. List all such integer pairs that give product $9$.
$$-1,-9$$ $$-3,-3$$
Calculate the sum for each pair.
$$-1-9=-10$$ $$-3-3=-6$$
The solution is the pair that gives sum $-10$.
$$a=-9$$ $$b=-1$$
Rewrite $x^{2}-10x+9$ as $\left(x^{2}-9x\right)+\left(-x+9\right)$.
$$\left(x^{2}-9x\right)+\left(-x+9\right)$$
Factor out $x$ in the first and $-1$ in the second group.
$$x\left(x-9\right)-\left(x-9\right)$$
Factor out common term $x-9$ by using distributive property.
