We know that According to Algebra of Real Functions :
If f and g are two real functions which are defined under the same domain then (f - g)(x) = f(x) - g(x)
Given f(x) = x² - 3x - 9 and g(x) = x - 7
⇒ (f - g)(x) = f(x) - g(x)
⇒ (f - g)(x) = x² - 3x - 9 - (x - 7)
⇒ (f - g)(x) = x² - 3x - 9 - x + 7
⇒ (f - g)(x) = x² - 4x - 2