Plotting f(x,y,z)=0?

[byoung]

Roadrunner and Albert, thanks for those solutions; both look like they should work. I'll probably need a few tries to get the fsolve syntax correct (syntax errors have become my nemesis as I get used to the Prime after 40 years of other HPs).

Albert, it's been too long since I've studied trig - I would never have thought of those substitutions!

Thanks again to both of you.