help please

f(g(x)) = f(Bx + A) → A(Bx + A) + B → ABx + A² + B

g(f(x)) = g(Ax + B) → B(Ax + B) + A → BAx + B² + A

f(g(x)) - g(f(x)) = A² - B² + B - A → (A - B)(A + B) - (A - B) → (A - B)(A + B -1) 

so we must have (A - B)(A + B -1) = B - A

Since B ≠ A we can divide both sides by A - B to get A + B - 1 = -1, so A + B = 0

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Thanks alan!!