I misspoke in class today ...
We calculated the potential due to a semicircle of charge, that was OK. I forgot about the line segments ... they do not cancel each other out, since they are both positive, and potential is a scalar (so there is no 'opposing direction').
What you need to do is superimpose on the semicircle result the potential due to two line charges a distance r away (along the line axis). Since both line segments are the same, find the result for one line and double it ...
See, e.g., PH106, F08, HW3, Q5.
Since I misspoke in giving you a hint, I won't count off if you miss the line bits.
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