since XY is congruent to YZ, that means they are equal to each other.
a circle is 360°. we already know XQZ is 199, so we just need to subtract it from 360
360-199 = 161
so arc XYZ is 161. again, since XY=YZ, we know that half of 161 is equal to XY and YZ
161/2= 80.5
YZ= 80.5°