Philip P. answered 10/09/19
Affordable, Experienced, and Patient Geometry Tutor
It looks as if point y lies on the line segment xz and you asked to find the length of yz
x-----------y-------------------z
By the Segment Addition Postulate:
xy + yz = xz
6 + 2a = 42
Solve for a. Once you have the value of a, yz = 2a.