14. 阅读下列材料：
将$x^2 + 2x - 35$分解因式时，我们可以按下面的方法解答.
解：竖分二次项与常数项，得$x^2 = x · x$，$-35 = (-5) × (+7)$.
交叉相乘，验中项，得，$7x - 5x = 2x$.
横向写出两因式，得$x^2 + 2x - 35 = (x - 5)(x + 7)$.
我们将这种用十字交叉相乘分解因式的方法叫做十字相乘法.
乘法原理为：若$ab = 0$，则$a = 0$或$b = 0$.
试用上述方法和乘法原理解下列方程：
(1) $x^2 + 5x + 4 = 0$；
(2) $x^2 - 6x - 7 = 0$；
(3) $x^2 - 6x + 8 = 0$；
(4) $2x^2 + x - 6 = 0$.

(1) 分解因式，得$(x + 1)(x + 4) = 0$.$\therefore x + 1 = 0$或$x + 4 = 0$.$\therefore x_1 = -1$，$x_2 = -4$
(2) 分解因式，得$(x + 1)(x - 7) = 0$.$\therefore x + 1 = 0$或$x - 7 = 0$.$\therefore x_1 = -1$，$x_2 = 7$
(3) 分解因式，得$(x - 2)(x - 4) = 0$.$\therefore x - 2 = 0$或$x - 4 = 0$.$\therefore x_1 = 2$，$x_2 = 4$
(4) 分解因式，得$(2x - 3)(x + 2) = 0$.$\therefore 2x - 3 = 0$或$x + 2 = 0$