Build a vector of a given length from a given point in a certain direction

Please put your right mind on the task (see figure): ♪ There's Cam, she's rotating around the coordinates center. ♪ There's a Marker Marker. His coordinates are known as the cell's coordinates. He's stable.
We need to find a point on the circumference around the marker that would lie on the cut from the marker to the camera (or vice versa). Simply put, I need the vector coordinates long in the radius of the circumference built from the marker towards the camera.
I've only thought of it before the vectors were recalculated to get a vector from the camera to the marker. Next, it's only a pumping method until it's a good one.
If possible, I want to meet the challenge without matrix and trigonometry for productivity reasons.

L^2 = (x2x1)^2 + (y2t1) ^2
x3 = x1 + R/L * (x2  x1)
similar to y3