Since they are vertical angles, that means that ∠AOB+∠BOZ=180 and ∠ZOX+∠AOX=180 too. Next, ∠AOB+∠AOX=180 (supplementary angles), as well as ∠BOZ+∠ZOX.
∠AOB+∠AOX=180
∠ZOX+∠AOX=180
Subtracting the first equation from the second, we get ∠ZOX-∠AOB=0. Adding ∠AOB to both sides, ∠AOB=∠ZOX=6x+5=80. Subtracting 5 from both sides, we get 6x=75 and x=12.5 by dividing both sides by 6