How do I get the. Length of the 15 mm line, use 15 as reference as I didn't not find it, ur help will be appreciated

Hi, I hope you are doing well!
It looks like you have the center-to-center dimension of two cylinders, which is helpful. However, this dimension may not be visible in the image you shared due to the photo zoom. To ensure accuracy, you have to add this dimension to the model. Once added, the line length will automatically be specified. I've indicated where to add this dimension in the attached images. Let me know if you have any questions!

If I understand your question correctly, you are trying to find the length of a line that is 15 mm long, but you don't have a reference measurement to compare it to. If that's the case, you can use a ruler or other measuring tool to find the length of the line.

Place the ruler or measuring tool next to the line, with one end of the ruler lined up with the start of the line. Then, read the measurement at the end of the line to determine its length.

If you don't have a ruler or measuring tool, you can try to estimate the length of the line by comparing it to other objects of known size, such as a standard size credit card or a sheet of paper. However, keep in mind that this method is less accurate than using a measuring tool.

You should pay attention if the 15mm distance corresponds to a VIRTUAL edge (in the center of the circle that then physically disappears due to the slot) or to a REAL edge (the one that materializes after cutting that slot). I show you both possibilities in a top view.
If it is a virtual edge, it is logical that later "you will not find it" in the 3D solid.

cylindrical figures always relate to other geometries by their center axis, if you want a specific location off the set of that axis, create your own work feature (vertices, plane, axis, or even UCS) then project those features through a sketch environment, and use them to apply or measure distances...