2x-0.4*75%=2.1 ；　2x=2.1+0.4*75%；2x=2.1+0.4*0.75；2x=2.1+3；x=5.3/2；x＝2.65　　　80％*【x-4】=3.2；x-4=3.2/0.8；x-4=3.2/0.8；x=4+4；x＝8