Bonjour,
a)x²-4+(x-2)(x+1) = x²-2² + (x-2)(x+1) = (x-2)(x+2) + (x-2)(x+1) = (x-2)[(x+2)+(x+1)] = (x-2)(x+2+x+1) = (x-2)(2x+3)
b)3x²-12x+12 = 3(x²-4x+4)
c)x²+3x+(x+3)² = (x+3)x + (x+3)(x+3) = (x+3)[(x+3)+x] = (x+3)(2x+3)
d)(x+1)(x+2)-(3x+6) = (x+1)(x+2) - 3(x+2) = (x+2)[(x+1)-3] = (x+2)(x-2) = x²-4