解：?$(1)$?∵?$y_{1}=y_{2}$?，∴?$-x+3=2x-3$?
移項(xiàng)，可得?$3x=6$?
系數(shù)化為?$1$?，可得?$x=2$?
∴當(dāng)?$x$?取?$2$?時，?$y_{1}=y_{2}$?
?$(2)$?∵?$y_{1}$?的值比?$y_{2}$?的值的?$2$?倍大?$8$?
∴?$(-x+3)-2(2x-3)=8$?
去括號，可得?$-x+3-4x+6=8$?
移項(xiàng)、合并同類項(xiàng)可得?$5x=1$?
系數(shù)化為?$1$?，可得?$x=0.2$?
∴當(dāng)?$x$?取?$0.2$?時，?$y_{1}$?的值比?$y_{2}$?的值的?$2$?倍大?$8$?
