hey there I’m just a begginner with grasshopper and I’m trying to manipulate these curves in grasshopper. Any advice on the best way to trim the extended arm of this semi-circle and join it into a new curve to follow the floor plans. Thanks
I didn’t see your file, anyway, why do you want to do this by using GH?
IMHO, simply doing this in Rhino is the best straightforward way.
It’s for an assignment where they are encouraging us to model from grasshopper rather than using rhino and defining curves
I have to agree with @HS_Kim though. Seems like an inefficient way to use grasshopper. Especially having to plot out each point in the plans. Rhino would be your best bet in copying plans. And also taking into account that many plans are not accurately displayed when scaled anyways throws a wrench in this to. (Personal experience.) 
This makes me think of the older days where you had to plot coordinate points with the keyboard in CAD software to draw lines. Prior to the “point and click” we all love? now 
