The problem you've provided is:

$f(x) = -x^2, \text{ find } x \text{ when } f(-5)$

To solve this, we simply need to substitute $x = -5$ into the given function $f(x) = -x^2$.

Step-by-Step Solution:

1. The function is $f(x) = -x^2$.
2. Substituting $x = -5$ into the function:

$f(-5) = -(-5)^2 = -(25) = -25$

Thus, $f(-5) = -25$.

Would you like more details or explanations on any specific part?

Here are 5 related questions you might find useful:

1. What is the general form of a quadratic function?
2. How do you find the vertex of a quadratic function like $f(x) = -x^2$?
3. What are the effects of negative coefficients on parabolas?
4. Can you explain how to graph quadratic functions?
5. What are the roots or solutions of $f(x) = -x^2$?

Tip: When evaluating a function, always follow the order of operations (PEMDAS), particularly when dealing with negative signs and exponents.