negative reciprocal, easy

Hi,

I have a slope of:

-4
-------
2

I need to get the negative reciprocal of it to find its perpendicular.
If I have a piece of paper and a pencil its easy to draw out the
negative reciprocal line. But I need to make a C++ function out of
this, and I'm not sure when flipping the fraction where if at all to
put the negative/postitive sign, on the new numerator or denominator?

Thanks for any help

.

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