Sinus issues

1: sin(2a)=f(x)
2: sin^2(a)=g(x)

Any way to rewrite those two so the left hand sides becomes the same, without introducing any trigonometric term (sin, cos, tan) nor any dependence on a to the right hand sides?

(If anyone wants the physical background, I'm stuck on calculating the initial velocity for a thrown item where the height and length of the throw is known, but the angle isn't.)

When I have sinus issues, I rinse them with salt water.