Answers: (a) 2x
(b) 12
(c) x² - 36

Domain: x ≠ 6 (-∞, 6) ∪ (6, ∞)
Explanation:
(f + g)(x) means "f(x) + g(x)"
x + 6 + x - 6 = 2x
(f - g)(x) means "f(x) - g(x)"
x + 6 - (x - 6)
x + 6 - x + 6 = 12
(fg)(x) means "f(x) × g(x)"
(x + 6) × (x - 6)
x² - 6x + 6x - 36 = x² - 36
(f/g)(x) means "f(x) ÷ g(x)"
(x + 6) ÷ (x - 6) =

Domain: The denominator cannot be zero, so
x - 6 ≠ 0
x ≠ 6