`y'=\frac{[x^2-(m+1)x+2m-1]^'.(x-m)-[x^2-(m+1)x+2m-1].(x-m)^'}{(x-m)^2}`
`y'=\frac{(2x-m-1).(x-m)-[x^2-(m+1)x+2m-1]}{(x-m)^2}`
`y'=\frac{2x^2-mx-x-2mx+m^2+m-(x^2-mx-x+2m-1)}{(x-m)^2}`
`y'=\frac{2x^2-mx-x-2mx+m^2+m-x^2+mx+x-2m+1}{(x-m)^2}`
`y'=\frac{x^2-2mx+m^2-m+1}{(x-m)^2}`