Let $\alpha_1,\alpha_2,....,\alpha_7$ be the roots of the equation ${x^7} + 3{x^5} - 13{x^3} - 15x = 0$ and $|{\alpha _1}| \ge |{\alpha _2}| \ge \,...\, \ge \,|{\alpha _7}|$. Then $\alpha_1\alpha_2-\alpha_3\alpha_4+\alpha_5\alpha_6$ is equal to _________.