The expression shown in the image is $\sqrt{-36}$.

Since the square root of a negative number is not defined in the set of real numbers, we must consider complex numbers. In the realm of complex numbers, the square root of a negative number involves the imaginary unit $i$, where $i = \sqrt{-1}$.

Thus:

$\sqrt{-36} = \sqrt{36} \times \sqrt{-1} = 6i$

So, the answer is $6i$, where $i$ is the imaginary unit.

Would you like further details on this, or do you have any questions?

Here are five related questions to further explore this topic:

1. What is the square root of other negative numbers like $\sqrt{-25}$?
2. Can the square root of any negative number be simplified similarly?
3. What is the definition of the imaginary unit $i$?
4. How do we perform operations (addition, multiplication, etc.) with imaginary numbers?
5. Can real and imaginary numbers be combined in other ways to form complex numbers?

Tip: The square of the imaginary unit $i$ is $i^2 = -1$, which is fundamental in calculations involving complex numbers.