Answer:
There is no error in the visible part of the subtraction provided. If I'm not mistaken though, I believe I see the start of a truncated equals sign on the right. I assume by this that you are not including the full image.
As I said though, there is no error in the image provided. Completing the statement we get:
(x² - 4x + 3) - (x² + 7x - 9) = -11x + 12
