## Equation of Curve Reflected in Line y=k

What is the equation of the curve
$y=f(x)$
after reflection in the line
$y=k$
?
It is
$-(y-2k)=f(x) \rightarrow y=2k-f(x)$
.
For example
$y=x^2+2x$
when reflected in the line
$y=3$
becomes
$-(y-6)=x^2+2x \rightarrow y=6-x^2-2x$
.