Angles that are across from each other are equal, so Z = 66, then you know that when you have a line, it equals 180 degrees, so you do 180-66=15x-6 and solve for x, when you do this, you get 8 as the answer.
180-66=15x-6
(+6) 114=15x-6 (+6)
(/15) 120=15x (/15)
8=x