(-5)x(+7又3分之1)+(+7)x(-7又3分之1)-(+12)x7又3分之1
=-5x7又3分之1-7x7又3分之1-12x7又3分之1
=-(5+7+12)x7又3分之1
=-24x7又3分之1
=-24x7-24x3分之1
=-168-8
=-176