Since the side of the square frame is 15x, the area of the frame in whole would be:
(15x)² = 225x²
Now if we subtract out the circle mirror with a radius of 5x, we get:
225x² - πr²
225x² - π(5x)²
225x² - π25x²
We can factor out a 25x² and get:
25x² (9 - π)