the 2 interior angles add to the exterior angle
26+13=x
39=x
x=39°
because
remember, all angles add to 180
26+13+BCA=180
39+BCA=180
minus 39 both sides
BCA=141
look, we want x
BC is a striaign line which is 180
BCA+x=180
141+x=180
minus 141 from both sides
x=39°