Le 93 svp je comprend pas aider moi svp

a =(x+1)(x+3) -5(x²+6x+9)
a = (x+1)(x+3) -5(x+3)²
a =(x+3)(x+1-5x-15)
a =(x+3)(-4x-14)
a = -2(x+3)(2x+7)

b = 16x²-4 -(4x-2)(4x+4)
b =(4x-2(4x+2) -(4x-2)(4x+4)
b =(4x-2) (4x+2-4x-4)
b =(4x-2)(-2)
b = -2(4x-2)
b = -4(2x-1)

c =(2x+5)² -(5x-1)²
c =(2x+5-5x+1)(2x+5+5x-1)
c =(-3x+6)(7x+4)
c =3(-x+2)(7x+4)