The expression in the image is:

$\sqrt{x^2 - 6x + 9}$

To simplify this expression, we first notice that the quadratic expression inside the square root can be factored. The expression $x^2 - 6x + 9$ is a perfect square trinomial, which factors as:

$x^2 - 6x + 9 = (x - 3)^2$

Thus, the expression simplifies to:

$\sqrt{(x - 3)^2} = |x - 3|$

The absolute value is needed because the square root of a square always yields the non-negative result.

Therefore, the simplified form of the expression is:

$|x - 3|$

Would you like more details or have any questions?

Here are 5 related questions you might find helpful:

1. How do you determine if a quadratic expression is a perfect square?
2. What are the steps to factor a quadratic trinomial?
3. Why is the absolute value needed after simplifying a square root of a squared term?
4. How would this expression change if it was under a cube root instead of a square root?
5. What happens to this expression when you substitute specific values of $x$?

Tip: Always check for perfect square trinomials when simplifying expressions under a square root, as they lead to clean simplifications.