What do you call the start of the ther?

x = 10

Explanation:

Based on the theorem of angles formed by two intersecting chords, thus:

Angle formed/vertex angle = ½(sum of intercepted arcs)

Angle formed/vertex angle = 60°

Sum of intercepted arcs = 55 + 5x + 15 = 5x + 70

Therefore:

60 = ½(5x + 70)

Multiply both sides by 2

60*2 = 5x + 70

120 = 5x + 70

120 - 70 = 5x

50 = 5x

Divide both sides by 5

50/5 = x

10 = x

x = 10