This is what it shows for the solution. Hope this helps.
C = 1/2(a - b), where a is biggest arc and c is outside angle
51 = 1/2(160-x)
51×2 = 1/2×2 (160-x)
102 = 160 - x
102-160 = 160-160 - x
-x = -58
x = 58
I have no clue let me do the work and I’ll comment it under here