Draw line segments ST and RQ. Triangle RUQ and SUT are similar, because they are in a circle. Now, you use similar triangles to get that QU/UT=RU/SU so 2 = 12/SU so SU = 6, or y = 6.
Note: This is generealized by the Power of a Point, which states that if you have two chords that intersect, then you can do the same thing and it will show that QU*SU=RU*TU. (You can change the variables but the similar triangles stay the same).
