Finding a previous value from a calculation

Unity Engine
1

I have a very simple question.

var fw = Quaternion.Euler(originalValue, 0, 0) * Vector3.forward;
var yValue = fw.y;

Using math, how can I get the value of ‘originalValue’ if I only know the value of ‘yValue’?

2

Impossible with only the y value
if you have the full ‘forward’ vector (both y and z), you can use

Mathf.Atan2( Vector3.Dot( Vector3.right, Vector3.Cross( Vector3.forward, forward ) ), Vector3.Dot( Vector3.forward, forward ) ) * Mathf.Rad2Deg

(but note that if your original x was (eg.) 340, you get -20 out, because they are the same angle withing 260 degrees)