原式=2a-2a²+2-8a^3+3a-21a²+6a^3
=-2a^3-23a²+5a+2
=-2×1/8-23×1/4+5×1/2+2
=-1/4-23/4+5/2+2
=-6+2+5/2
=-4+2.5
=-1.5