原式 = [x^2 + 6 + 5x]^2 - x^2 - 3x^2 = [x^2 + 5x + 6]^2 - 4x^2
= (x^2 + 7x + 6)(x^2 - 3x + 6) = (x+1)(x+6)(x^2 - 3x + 6)