The question displayed asks to simplify the expression involving square roots of negative numbers:

$\sqrt{-9} - \sqrt{-4}$

Step-by-step Solution:

1. Handle each square root separately:

   • $\sqrt{-9}$ can be rewritten using the imaginary unit $i$, where $i = \sqrt{-1}$. $\sqrt{-9} = \sqrt{9} \cdot i = 3i$
   • Similarly, $\sqrt{-4}$ can be simplified: $\sqrt{-4} = \sqrt{4} \cdot i = 2i$

2. Substitute back into the expression: $3i - 2i = (3 - 2)i = 1i$

Thus, the result is: $\boxed{i}$

Would you like more details or have any questions on this problem?

Here are 5 related questions you might find helpful:

1. How do you multiply complex numbers?
2. What is the square of an imaginary number $i$?
3. How do you simplify powers of $i$ (e.g., $i^3, i^4$)?
4. What is the geometric representation of complex numbers?
5. How do you add and subtract complex numbers?

Tip: Whenever you see the square root of a negative number, remember to express it in terms of $i$, the imaginary unit.