Check the picture below.
notice, on the first 2 lines, we used the quadrilateral conjecture.
on the lines 3 and 4 we used the inscribed angle theorem.
line 5, if we add those 3 arcs, we end up with 427°, mind you that the surplus is arcYW or "b", since it's added twice by XW and YZ.
line 6, if we subtract a full circle from it, we have the surplus.
line 7, we simply subtract YW from XW, leaving the leftover of arc "a".
