Mixed Coordinate conversion - ra from long and dec

Can any one help me with a bit of math.
I would like to get right ascension given the celestial longitude and
declination of an object. Yes, that's right I do mean mixed co-ordinate
systems. I know the equations that convert ecliptical to equatorial
co-ordinates e.g. celestial long and lat to ra and dec (and vica versa)
but I'm darned if I can get the trig re-arranged to give ra as a
function of celestial longitude and declination. Not so worried about
celestial lat, but it could be part of it.

It might sound odd, but I really do need to solve this. Hope this is
right place to try.
Dave

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