All the angles in a circle equal to 360 degrees. So basically just add up all the known values: 23+23+15=61 and then subtract that from 360: 360-61=299.
You can also do 23+23+15+x=360. Adding it all becomes 61+x=360, subtract 61 from both sides of the equation, and you get x=299.
(Something to help remember is that the angle is obtuse, meaning it’s automatically more than 90 degrees, hope this helps!)