若y1=x-1,y2=2x+1,且y1-3y2=0,则x=_____,y1+y2=_____ 怎么算.

问题描述:

若y1=x-1,y2=2x+1,且y1-3y2=0,则x=_____,y1+y2=_____ 怎么算.