(-x-5)^2 - 2(x+1)(x+2)
= (-x-5)(-x-5) - 2(x+1)(x+2)
= (x^2 + 10x + 25) - 2(x^2 + 3x + 2)
= x^2 + 10x + 25 - 2x^2 - 6x - 4
= -x^2 + 4x + 21.