Bonjour,
1) x² + 8x + 15 = (x² + 8x + 16) - 1
= x² + 2(x)(4) + 4² - 1
= (x+4)²-1
2) (x+3)(x+5) = x²+5x+3x+15 = x²+8x+15
3) A(0) = 0² + 8(0) + 15 = 15
A(-3) = (-3 + 3)(-3+5) = 0
A(-4) = (-4+4)²-1 = -1
A(-5) = (-5+3)(-5+5) = 0