SOS Need help solving the length

[avergel]

Geometry

Can you please explain the tangent secant theorem when the vertex is on the circle? I'm confusing it with the one where it is not on the circle.

 

[stapel]

Can you please explain the tangent secant theorem when the vertex is on the circle? I'm confusing it with the one where it is not on the circle.

Please reply with the exact text of the theorem in question. Thank you!