$解:①∵B=x^{2}-4x+6=(x-2)^{2}+2$
$代數(shù)式C參照代數(shù)式B取值后移,相應(yīng)的后移值為2$
$∴C=(x-2-2)^{2}+2=x^{2}-8x+18$
$②∵代數(shù)式ax^{2}-7x+2b參照代數(shù)式$
$2x^{2}-3x+c取值后移,∴a=2$
$∴代數(shù)式2x^{2}-3x+c=2(x-\frac{3}{4})^{2}+c-\frac{9}{8}$
$代數(shù)式ax^{2}-7x+2b=2x^{2}-7x+2b$
$=2(x-\frac{7}{4})^{2}+2b-\frac{49}{8}$
$∴代數(shù)式ax^{2}-7x+2b參照代數(shù)式$
$2x^{2}-3x+c取值后移,相應(yīng)的后移值為1$
$∴2b-\frac{49}{8}=c-\frac{9}{8}$
$∴2b-c=-\frac{9}{8}+\frac{49}{8}=5$
$∴a+2b-c=2+5=7 $