Derivative of (u/v)

f(x)= y= (x^2-6x+9)/(5-x)

use derivative formula as

d(u/v)= [v*d(u)-u*d(v)]/(v^2)
put u=(x^2-6x+9) and v= (5-x)

hence derivative r
d(u)= 2x-6 and d(v)= (-1)

so
f'(x)= {[(5-x)(2x-6)]-[(x^2-6x+9)(-1)]}/ (5-x)^2
= {[10x-30-2x^2+6x]-[6x-9-x^2]}/(25+x^2-10x)
= {10x-21-x^2}/(25+x^2-10x)
= {(-1)(x^2-10x+21)}/(x-5)^2
= [(3-x)(x-7)]/(x-5)^2