To calculate \( 75 - \sqrt{-45} \), we first simplify \( \sqrt{-45} \).
- Recall that \( \sqrt{-a} = i\sqrt{a} \), where \( i \) is the imaginary unit.
- So, \( \sqrt{-45} = \sqrt{45}i \).
- Now, simplify \( \sqrt{45} \):
- \( \sqrt{45} = \sqrt{9 \times 5} = 3\sqrt{5} \).
Thus, \( \sqrt{-45} = 3\sqrt{5}i \).
Now we can substitute this back into the expression:
\[ 75 - \sqrt{-45} = 75 - 3\sqrt{5}i. \]
The result is:
\[ \text{The difference is } 75 - 3\sqrt{5}i. \]