The arc measures shown are 138 and 58. Subtract the arc measures and divide by 2 to get the answer
(138-58)/2 = 80/2 = 40 degrees
The general formula is
(A - B)/2
where A and B are the arc measures. The value of A is always the larger arc measure. This formula works for two secant lines as this diagram shows, and it also works for a secant line and tangent line, and lastly it works if both lines were tangent.
