i get mABC = 270°
Explanation:
This is fairly straight forward once you the right angle symbol for the arc AC. Also realizing the 148° for mAB is not needed for this question. The 148° for mAB would be useful if you wanted to calculate arc mBC.
Remember there are 360° in a circle.
The arc mAC is 90°
The arc mABC is the rest of the circle. is the arc from A to B to C
arc ABC + arc AC is the full circle 360°
mABC + mAC = 360° mAC = 90°
mABC + 90° = 360° - 90° from both sides
mABC = 360° - 90°
mABC = 270°