Use the imaginary number i to rewrite the expression below as a complex number Simplify all radicals. 15-sqrt (-72) square

To rewrite the expression $15 - \sqrt{-72}$ as a complex number, we need to simplify the square root of the negative number.

First, let's rewrite $\sqrt{-72}$ using the imaginary unit $i$:
$$\sqrt{-72} = \sqrt{72} \cdot \sqrt{-1} = \sqrt{72} \cdot i$$

Next, we simplify $\sqrt{72}$:
$$\sqrt{72} = \sqrt{36 \cdot 2} = \sqrt{36} \cdot \sqrt{2} = 6\sqrt{2}$$

So,
$$\sqrt{-72} = 6\sqrt{2} \cdot i = 6i\sqrt{2}$$

Now, substitute this back into the original expression:
$$15 - \sqrt{-72} = 15 - 6i\sqrt{2}$$

Thus, the expression $15 - \sqrt{-72}$ rewritten as a complex number is:
$$\boxed{15 - 6i\sqrt{2}}$$