I think it’s C but it could be D.. i would go with C tho
We have to know the angle that is not labelled. (lets call it y)
9x + y= 180
and the sum of the angles of a triangle is 180
5x + (9+x) + y=180
2 simultaneous equations
What you do is you rearrange and simplify both equations to have them in terms of y, and then graph them. If it asked for a solution, their point of intersection would be the solution.
I think it is B
hope this helps