(x+2)2 - (2x+1)2
= (x2+4x+4)−(4x2+4x+1)
= x2 + 4x + 4 - 4 x2 - 4x - 1
= -3 x2 + 3
= 3 - 3 x2
= 3(1 - x2)
= 3(1 + x)(1 - x)