The expression in the image is:

$4 - 3\sqrt{-1}$

Here, $\sqrt{-1}$ represents the imaginary unit, often denoted by $i$, where $i = \sqrt{-1}$. Substituting $i$ for $\sqrt{-1}$, we can rewrite the expression as:

$4 - 3i$

This expression is in the form of a complex number, where $4$ is the real part, and $-3i$ is the imaginary part.

Final Answer:
The simplified form of the expression is:

$4 - 3i$

Would you like further details or have any questions?

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Here are some related questions to expand on this concept:

1. What is the significance of the imaginary unit $i$ in complex numbers?
2. How do you add or subtract complex numbers?
3. What is the modulus (magnitude) of the complex number $4 - 3i$?
4. How can you multiply or divide complex numbers?
5. How is the complex plane used to visualize complex numbers?

Tip: When dealing with square roots of negative numbers, always consider if they involve imaginary units.