So you know that a square has 360 degrees total. A triangle would have half of that which is 180. Even tho it’s not a right angle you can use algebra to find it. What you would need to do is add all the angles together and set it equal to 180. Then use algebra to find the x.
13x-11+ (18x-15) + (4x+1) = 180
Combine like terms
35x-25=180
Add 25
35x= 205
Divide by 35
x=5.857
C is the closest answer :)