(a - b)^2 - a(a + 2b) = a^2 - 2ab + b^2 - a * a - a * 2b = a^2 - 2ab + b^2 - a^2 - 2ab = a^2 - a^2 - 2ab - 2ab + b^2 = -4ab + b^2.
(a - b)^2 - a(a + 2b) = a^2 - 2ab + b^2 - a * a - a * 2b = a^2 - 2ab + b^2 - a^2 - 2ab = a^2 - a^2 - 2ab - 2ab + b^2 = -4ab + b^2.