# What is the equation of the line normal to  f(x)=-x/(x+4) at  x=6?

y = 25 x $-$753/5.
Denoting f(x) by y, the equation simplifies to $-$1 + 4 / ( x + 4 ).
The point under reference is P ( 6, $-$3/5 )
$\frac{d}{\mathrm{dx}}$y = $-$4 / ( x + 4 )^2.
The slope of the normal at P is $-$1 / $\frac{d}{\mathrm{dx}}$y at P = 25.
So, its equation is y + 3/5 = 25 ( x $-$6 )..