Find the Domain y = square root of x-9

$y = \sqrt{x - 9}$

Set the radicand in $\sqrt{x - 9}$ greater than or equal to $0$ to find where the expression is defined.

$x - 9 \geq 0$

Add $9$ to both sides of the inequality.

$x \geq 9$

The domain is all values of $x$ that make the expression defined.

Interval Notation:

$[9, \infty)$

Set-Builder Notation:

${x | x \geq 9}$