How do I remove these back lines?

Whenever I render this I get these black lines and I don’t know how to remove them.

Maybe your geometry isn’t connected there and rays are slipping through the cracks? You might try to either overlap it a bit more (make the geometry intersect), model it as a single water-tight mesh, or perhaps use a boolean operation to join the parts. Also giving the surfaces some thickness may help.

Hard to tell without a file to look at.