Find the distance of point from a line passing through a circle

Question

I am looking for a solution to the problem shown in the below image

Problem image

Known values:

1. Radius of the circle = r
2. Length of the green line = 0.3 * r
3. Distance of dot(point) from the center of the circle = a

Value to find:

1. Length of the red line (X)

Please let me know if any additional information is required. Thank you.