Remark
I would have had the answer a whole lot sooner if I would have read the question properly. The figure in the circle is called a cyclic quadrilateral. It has the odd property that the angles that are opposite each other add up to 180o.
So DEB + DCB = 180o
DEB = 180 - 87
DEB = 93o
Note: The arcs marked 60 and 76 have nothing whatever to do with this problem.
X = - 7 - y ( - 7 - y ) y = 12y = - 3 or y = - 4x = - 7 - ( - 3) or x = - 7 - ( - 4)( - 4, - 3)( - 3, - 4)
What midpoints are you wanting to measure?