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  
\[-(y-6)=x^2+2x \rightarrow y=6-x^2-2x\]
.

reflection

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