First solve for the other angles.
You know one angle is 15º.
The one across from it is also 15º since it is a vertical angle to the known 15º angle.
Both of those together gets 30º.
The whole thing is 360º, so just subtract 30 from 360 and divide.
360 - 30 = 330
Do the same for the 15x angle.
The one under/across to it is a vertical angle so is also 15xº.
Now you are left with 30xº = 330.
Just divide now.
330 / 30 = 11
x = 11
15x = 165
Checking your work:
15x = 165
15x = 165
15
15
Those are the angles.
Add those numbers together and make sure it adds up to 360º.
165 + 165 + 15 + 15 = 360
So this is correct.