The measure of the external angle is half the difference of the measures of the arcs it intercepts.
... (5x-10) = (1/2)(195 - (8x+17))
... 10x - 20 = 178 - 8x
... 18x = 198
... x = 11
Arc EHF = 195° + (8·11 +17)° = 300°
The appropriate selection is
... B. 300
