Quote from Aristocrat

Arrived at this equation after I'm halfway done with a problem and can't figure out a way to isolate theta.

Maybe you messed up somewhere earlier. Any chance that you might post the original equation?

It's just the usual parameterization of a particle on an earth trajectory (no drag forces or anything, just the gravitational potential field).

x = vcos(theta)t
y = vsin(theta)t - .5at^2