load the attachment https://www.geogebra.org/m/KrYWuNEv see the description of the construction slider - $alpha$ -select the angle slider - point P - ruler with a socket, point Q must be line n

Whether you are from the previous understand how it works proportions of angles, or that you explain step by step?

Attachments https://www.geogebra.org/m/HQm7WwFk on the ruler $AB\infty{_1}\infty{_2}$ , raises divider ADC where AB + AB = AC, ruler sets the angle $\alpha$ semi-line ruler $B\infty{_1}$ sliding on point E , the point A of ruler slides semi-line l , when point C is on the line n , we get the radius of the circle , we get the angle $\beta$ we have solved the tricection of any angle Look at the construction protocol , or find the error if there is ....

https://www.geogebra.org/m/CukhmEVy straightedge slip on the point B - line i divider FIG , slides on straightedge , after slipping point F straighte line BC , point G describes lokus1 section lokus1 and line k point J , when changing the angle, the point J must be manually set to the intersection