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!)
Because this is a fourth degree that will be tricky to factor as it is, we will do a u substitution. Let . We can now rewrite that polynomial in terms of u: . Filling into the quadratic formula we have . Simplifying down and or . That means that u = 8 or u = -1. But don't forget that we let , so we have to put x-squared back in for u. That gives us which simplifies down to . That also gives us . When we take the square root of -1, we have to use the fact that -1 = i^2, so we sub that in to get . All in all, your solutions are as follows: which is choice b.