Need help, circle theorem. https://imgur.com/a/ADb4qHm
1) The angle sum of a quadriateral is 360 degrees
2) the angle at a point is 360 degrees
3) When subtended from the same arc, the angle at the centre is twice the angle at the circumference.
In this case the angles are subtended from arc BC
Use facts 3 and 2 to get all the angles of the quadrilateral in terms of x and then use fact 1 to solve for x