Answer:
138°
Explanation:
You want the measure of the angle at two tangents when they intercept an arc of 42°.
Supplementary angles
The short answer is that the exterior angle x is the supplement of the measure of the arc:
x = 180° -42°
x = 138°
Exterior angle
An exterior angle where secants meet is half the difference of the arcs of the circle they intercept. Here, the secants have been located so the corresponding chord length between the near and far circle intercept points have degenerated to zero. That is, they are tangents.
The angle relation still holds:
x = (long arc - short arc)/2 = ((360° -42°) -42°)/2 = (360° -2·42°)/2
x = 180° -42° = 138°
Quadrilateral
The tangents, together with their associated radii form a quadrilateral. The angles at the tangents are 90°, and the total of all angles is 360°. This gives us the relation ...
x + 90° +42° +90° = 360°
x +42° = 180° . . . . . . . . . . . . . subtract 180°
x = 180° -42° = 138°
(We solved this with an extra step, so you could see the same "supplementary angles" relationship between x and 42°.)